I'm going to refuse to pay an artist after she sold some of her displayed work and didn't give us (the gallery) commission
That's a lot to swallow. Why do you feel that is fair?
The gallery relies on commissions to stay open.
I understand that the gallery relies on commissions. Did you inform her when you made the agreement to sell her paintings that a commission is required? You shouldn't withhold payment from her for the total amount of the sales and need to give her the money left after deducting your commission.
I'm going to sue her gallery. She's going to have to pay us back, that asshole.
Your frustration is valid, but I suggest you relax for a bit first before making such decisions. Being in the heat of the moment can lead to something you may regret! How about just paying her first to mend the relationship, and then discussing what went wrong to clear up misunderstandings?