How do you enforce Plasticity? I mean when is it set in stone of the path that the character is moving. Because your opponent thinks their character is going to square A, but it really has to go back to square B, because your character has Plasticity and they didn't see it or forgot about it. They always reply to me that their character will go to square C or go around my character instead.

When moving a character you technically should declare what squares you are moving through. Not what square you are starting from and stopping in, but each square for the movement. Your opponent then begins their movement going square for square. At any point that they become adjacent to your character you can inform them that they must stop due to Plasticity. If they then try to change the square they are moving into, that is technically cheating as that was not the square they designated for movement.

With that said, most players do not actually play this way and simply state where they are starting and stopping their movement.

Again though, there are no take backs in Clix. If you declare you are moving into square X during your movement then you are moving into that square period.