I'm going to deny my biological father contact with his grandchildren
Why would you deny him a chance to see his grandchildren?
Because he's a huge asshole and his wife is awful.
I am sorry you are feeling that way. It's okay to want to protect your children from someone that could hurt them. People can change though so you should be open to second chances.
Yeah, they're both terrible people that my kids don't need to see.
I think it's fine that you're not letting your kids see them if they aren't good people. It's important that they have good role models.