And surely the Red Skull isn't the only major Cap villain, right?
Well...it often seems that way, frankly.
For a movie, I'm sure they wouldn't have that big a deal building somebody else up, but if they haven't been in touch with him at all (which is what it sounds like), it doesn't seem like they were intending on using the Skull in the second movie anyway.
I think if he signed a contract, it shouldn't be an issue of "forcing" him to do it, and more a matter of living up to your obligations.
Strictly speaking, the contract is meaningless if he doesn't want to carry it out, beyond perhaps him owing them some financial penalty (depending on the details). Courts would never give specific performance in this area.
Not like it would come to that, anyway. The nature of the role makes it particularly easy to replace him.