If you are talking ranked Riot has already said Pick > Call order so you can report people for this. Just screen champion select and throw the imgur link in the report description at the end of the game.
OR
You can play versatile champions like Warwick, GP, Lee Sin and Lulu (They can pretty much play every role decently) so people randomly taking your role won't hurt you much.
OR
You can play versatile champions like Warwick, GP, Lee Sin and Lulu (They can pretty much play every role decently) so people randomly taking your role won't hurt you much.