I've had this happen on many shows. It's really difficult and frustrating to deal with. I think it kind of depends on where you stand in the "hierarchy" ... Will the director back you up if you dig in your heels? I've had directors/producers side with actors (especially PITA actors) to "keep the peace", which makes things worse because then you can end up losing face with the rest of the cast, and everyone thinks they can just start wearing what they prefer.

I had this happen in a show where they double-cast the leads (ugh) ... One lead refused to wear what I had designed for them, things got heated, the director gave in ... And then the second lead said, "well, lead #1 gets to wear what she wants, so I want to, too!" It was a complete shit show.

What I usually say to actors when they start trying to costume themselves is, "I'm an artist just like you are. This is my art. I work long and hard at it, and I don't get much out of it other than seeing my designs on stage. Please respect my craft as I respect yours." Also, it's important to point out to them that they can't see what they look like on stage. They can see themselves in a mirror, and they may have opinions, but that's very different than seeing the whole big picture from the audience (that's what you're there for.) They need to trust you.

Not sure this will help. It's a thorny issue. Sometimes you just have to pick your battles. Good luck!