I am sure they could force someone or else get some money back from that actor, especially if the actor is going to the competition. Hollywood lawyers are probably one of the worst in the world so I wouldn't be surprised if Marvel/Disney could get away with locking these actors to these many films in a contract. I don't think we even have any slightest clue how crazy and detailed these contracts get.
I'm also going by what Weaving said, he clearly said he would have to do it if they really wanted him and it was up to Marvel if they wanted to force someone to do something they want to do but contractually, they could if they really wanted to. In the end, it's just whether you want someone to throw the performance if you force them to do it.
I [signed a multi-picture deal] for Captain America. I think the tendency, with those films, would be to probably not bring a villain back. They might for The Avengers, but I didn’t think I’d be in Captain America 2 or 3. I don’t think Red Skull will be there. And it’s not something I would want to do again. I’m glad I did it. I did sign up for a number of pictures and I suppose, contractually, I would be obliged to, if they forced me to, but they wouldn’t want to force someone to do it, if they didn’t want to.
Elba has also been talking about being unhappy as Heimdall.
Speaking to The Telegraph last month, Elba spoke about the transition from playing Nelson Mandela in Mandela: Long Walk to Freedom to returning to the Marvel Universe as Heimdall:
"It was really weird...I’d just done eight months in South Africa. I came to England and the day I came back I had to do reshoots on Thor 2....And in the actual scene my hair was different, my…[exasperated sigh.]...“I was like, ‘This is torture, man. I don’t want to do this.’ My agent said: ‘You have to, it’s part of the deal.’ "
His feelings towards the Marvel movies didn't improve during the reshoots...
“I’m actually falling down from a spaceship, so they had to put me in harness in this green-screen studio. And in between takes I was stuck there, fake hair stuck on to my head with glue, this f***** helmet, while they reset. And I’m thinking: ‘24 hours ago, I was Mandela’. When I walked into the set the extras called me Madiba. I was literally walking in this man’s boots. [Within] six months, the crew, we were all so in love with this film we had made. I was him. I was Mandela, practically...Then there I was, in this stupid harness, with this wig and this sword and these contact lenses. It ripped my heart out."
And take it with a giant grain of salt but when sent a PS of him as GL, he did respond well.
Idris Elba ✔ @idriselba
Thank you baby love..i like this idea..."@HeyTaz: @idriselba you should set this up 😻😻😻😻😻😻 pic.twitter.com/htnwDK5OjD