To ban a user, hover over the comment and an X will appear in the upper right corner of the comment. Click it and choose the option Remove Comment and Ban User.
If you change your mind and the banned user promises to behave, you can choose to unban. Click the people like this link on the left sidebar of the page (it appears next to the number of fans as a link). A box pops up with a list of people who like this page. Click the top left arrow that says “People” and choose the filter Banned. This will show you the list of people who have been previously banned. You can click Unban to unban the user.