You can always close the individual topics...but a better idea may be to remove posting rights to that forum. To do that, go into the control panel to the UBB Forum module. Choose that forum from the Forum Configuration tab, and scroll down to Permissions. Under "Who Can Post Topics?" and "Who Can Reply?", you'll want to change those permission circles to Specific Permission Circles, and choose Admins or Super Admins.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .