When it comes to people who do relational cheating as in a marriage, I'm undecided. I do believe anyone can change. I think great people can go the wrong way and people of terrible character can turn and go the right way.
In my experience, only those as devastated by their own behavior over what it did to someone else will ever change. Obviously, a person cheated on hurts deeply...when the partner feels it as deeply there can be change. If they can't, then why would they change?
Faith also plays a role in this in my worldview. If you're a faithless or low on faith individual and commit to being faithful I think mountains can be moved.
As for other types of cheating, I would guess the above applies to it also.
Like Steve said once a person goes too far with anything the way back becomes almost but not entirely impossible.