If somebody wants to follow you, you can block them from doing that -- but you can't stop them from commenting on your Q&A. And if they bookmark your profile you can't even stop them from following you.
What you can do is flag any of their comments that you find offensive and the Mods will decide whether to remove them.
No, you cannot block someone on ask.com. There is a block button, but any intelligent person can figure out how to follow people without "following" them, even with the app. It's basically a gesture simply saying I don't like you.
You go to the persons profile and click block. But all it does it make it to where they cannot follow you. They can still answer our questions or comment on your answers if they see you have posted something.