What is keeping mages from controlling the world? How is economy, capitalism, business, and merchants affected? Can mages turn common items (stone, acorns) into gold? Can mages destroy currency systems by mass producing gold and making coins/money/currency/gold worthless?  What is stopping mages who control the weather, to cause draughts and hold crops hostage, asking huge ransoms to allow rain to fall? Why would people need carpenters if mages came build houses at the wave of a wand? Do mage chefs who create perfect food at will put bakeries and restaurants out of business? What about mages with unchecked, unlimited power? Do they exist? What do they do? Why? Your characters are not all-powerful gods, are they?  How do governments deal with mages like these?  Why is the Universe not 100% under control of one magic user? How do you keep magic out of the wrong hands? Define the wrong hands. Their enemies? The poor? Criminals? Are most magic users are heartless, and even the better ones are quite cold and rude? Is it just pretty much just peer pressure keeing them in check? Is anyone who wants too much power is frowned upon and alienated, and if they actually achieve too much power, the best mages gather up and try their best to kill that person?

* The poor? 

Most poor, side with the mages, and not the Guild, as most of the illegal magic arts, are in fact very helpful to poor families.

Like for example, Swamp Witches make medicinal teas and elixirs, but often they are elderly women who can not afford to cost of buying licenses and permits and certification, so they practice without paperwork, and thus their potions, may in fact be more powerful and work better than those made by healing mages in the hospitals. But hospitals are expensive and poor people can not afford them, so will usually sneak out to the Swamp Witch instead.

Well, this puts BOTH the poor families and the Swamp witches at risk of fines, imprisonment, or worse.

* Criminals? 

Many criminals become mages. And many mages, once targeted by the Justice Mages, are forced to live a life of crime, as their reputations are so damaged they can not live in normal society again.

Most drug dealers/chemists are mages, as are most moonshiners, most gang leaders, and most scam artists. 

The law has reached a point of being so corrupted, that many turn to criminal slums for safety from the Justice Mages.

Yep.

This is how it is in mine.

And to quote the MC, Quaraun, himself from the novel Kelim and the Necromancer:

>>>"Years? Multiple years to make one potion?"

>>>"Oh, yes. You weren't expecting me to wave a wand and go POOF! Were you?" Quaraun pulled out his wand as he spoke and waved it around over his head.

>>>"Well, yeah, kind of. You are a wizard, after all."

>>>"You been reading too many Fairy Tales, Boy. Real world magic, isn't like what they tell you in Fantasy novels."

>>>"But magic is real, right?"

>>>"Oh, yes. Magic is most certainly real. I just think, real magic, may not be quite so glamorous as you are imagining it to be."

>>>"So, no magic wands?"

>>>"No."

>>>"But you are holding one."

>>>"What? This?" Quaraun waved his rainbow wand around again. "This is nothing."

>>>"Than why do you have it?"

>>>"In case I need to kill someone."

>>>"Kill some. . . one?"

>>>"Yes."

>>>"With a little wooden stick?"

>>>"Yes."

>>>"How?"

>>>"Well, usually, I just wave it around while I talk, and when they least expect it, I ram it up their nose and through their brain. And POOF! Instant death by magic wand. No magic needed."

>>>"You're kind of violent, aren't you?"

>>>"The world is a violent place, Kelim. Of, course, I can also just do this."

>>>Quaraun spun around, aimed the wand, and a brilliant purple bolt of lightning shot out from the end of the wand and blew up the table on the other side of the room, leaving nothing but a smouldering piles of ashes where moments ago had been a table.

>>>"That. . . that.. how..?"

>>>Quaraun pointed to his 12 foot long hair. "World's longest hair."

>>>"So?"

>>>"It means, I am the world's most powerful wizard. More hair equals more power, Boy."

and later in the same scene:

>>>"You know what I do to rapists now?"

>>>"Uhm. . . do I want to know?" 

>>>"I jab my wand in their eye sockets, scramble their eyes and their brains to jelly. Serve it on toast. Absolutely marvellous. More death by magic wand, without any magic at all. It's so satisfying. I hate rapists."

>>>"You seem to like scrambling brains with your wand."

>>>"Oh yes. I've become addicted to it. Brains are such a delicacy."

>>>"Are all wizards like you?"

>>>"What do you mean?"

>>>"Uhm. . . you're. . . you're kind of crazy. And mean. And, you. . . well. . . you don't seem to care about anyone. Like, killing is just easy for you. And you eat brains!"

>>>"Ah. Yes. A side effect of spending a hundred years locked in a tower with no one but me, myself, and I to talk to. I find I'm much more suited to conversations with myself now, than I am conversations with Humans. And Humans are rather delicious. I would much rather eat a Human than talk to a Human."

and even later in the same scene:

>>>"You already said that one," Kelim pointed out.

>>>Quaraun glared at the boy. "You're interrupting me again."

>>>"You're repeating stuff. You listed off a few of those things more than once. . ."

>>>"Do you want my wand up your nose?" Quaraun pressed the end of his wand on Kelim's nose as he said this.

>>>"No," Kelim squeaked.

>>>"Good. Then shut up. You're the one invading MY privacy. If you don't want to listen to me pontificate, than you don't have to be here. You can turn your ass around and march right out that door. No one invited you here, Kelim. I have better things to do than waste my time with sex crazed ignoramuses like you."

>>>"I'm not a sex crazed ignor. . . ignor. . . whatever you just called me."

>>>"Ignoramus. It means you are too stupid to know the big words I use."

>>>"I. . . uhm. . ."

>>>"Now where was I? Oh yes!" 

My MC doesn't give 2-fucks what other people think of him, and he sees no issue with killing a Justice Mage that dares confront him. He is cold and rude and stopped caring what others thought of him centuries ago.

Yep. Same in mine as well.

Only it's the Guild's Justice Mage who do the shaming and humiliating. They make wanted posters and put them up all over the mage's home town, and any place the mage's friends, family, and clients live. They spread rumours and gossip and make up horrible les about the mage and do everything they can to slander, defame, humiliate, and ostracize him. They are known to incite local villages into angry mobs and, start rioting against the mage and his family. Many mages are dragged through the streets, stripped naked, and publicly beaten to death.

The sheer brutality and extreme levels of torture the Justice Mages do to any mage they turn on, is usually enough to scare most people out of ever becoming a mage.