At the moment you can only block someone on Ask.com if they are following you. If this is the case, you can click on their username or profile picture to go to their profile page. At that point you should see a link near the top, beside their profile picture, that says 'Block'. Click that, and that person will not be able to follow you anymore.
This does not necessarily prevent them from answering your questions or commenting on your answers, but it makes it more difficult.
If you continue to have problems, please don't hesitate to talk to Customer Service: