I hate when people take over what I'm doing and didn't ask me. I've made it a habit to give up and walk away, and leave them to whatever it was they took over.
Two of my examples --
A birthday party for a friend. I'm running the grill. The most important thing about grill is keeping up with everything. You can't let what you're grilling sit to long or it cooks uneven and burns. Basically time management. I'm there at the grill, and I just made a sweep(left to right) of the meat, applying another layer of marinade to cook on it and turning them over to have what I applied cook. I was then about to start over with another sweep of the grill because it had cooked long enough that it was time to do it again. I started, and a girl came over said I was doing it wrong. She started from the end I had just finished, and began to apply the marinade and turning them. I looked at her and said I had just did that and she ignored me, and I put down my tongs and brush, and walked away leaving her there at the grill. I did nothing else. I refused the rest of the day to cook anything else on the grill, and she and the others didn't understand why I wouldn't cook anything, and I told her that I was doing a good job of it and by her action she had deemed me not needed for the job so why should I do it? She did the grilling the rest of the night.
In a D&D 3.5 gaming session. I'm playing my character, and for once despite my aspergers, I thought I was doing a good job of it, and suddenly two of the people in my party and the DM were you can't do this and that in response to what I was having my char doi and then they started playing my char. I was like WTF. After a few minutes, got up and went to the front room of the house and didn't return to gaming. As far as I was concerned, I no longer had a character to play and wasn't needed in the game. I didn't play for the rest of the night, and rolled a new char before next session. They were a bit confused and after explaining to them as best I could what they did, they apologized, and said they wouldn't do it again.
This is my reaction pretty much. Anger with a little bit of confusion, and the attitude that you wanted it more than I did, so you can have it and be stuck with it.
As to what I would have done. I would have tossed the artwork. It wasn't my piece of art at that point. I would have gotten a new canvas and started all over. If they would have asked why, I would have just said that it's no longer my work, and I need a new canvas to work on. If they were still confused, I would possibly very bluntly and very unfriendly point out that someone else painted on my piece and ruined it, or I'd quit the course and ask for a refund. Worse case, if I was really offended, I'd cut and run, not ask for a refund, and just avoid the place, and badmouth the place, steer others away from it, write bad reviews about it on the internet. I guess it would just depend on how upset I felt about it.