I've read multiple blogs, etc. that have mentioned lemon oil as a good deterrent. You just get a very small amount and put it where you don't want them chewing. You can do a little more research on exactly how this works if you'd like.
Linus hasn't even noticed the baseboards in my room yet, so I haven't had to encounter this problem yet.
What I do know for sure is not to go for bitter apple spray like they use for dogs. A lot of rabbits not only don't mind it, but actually like it MORE.
Another thing you can do is make them a "faux baseboard" to chew on by geting some rabbit-safe wood that kind of mimics the width and look of your baseboards and attach it upright somewhere they can easily get to.