You can do this in two ways. First, you can block PM's from people of your choosing.
1. Click on Private Messages (left sidebar.)
2. Click on 'Banning.' (on taskbar below the two blue ones.)
3. In the pop-up window that appears (titled 'current ban list') you may enter the user name you wish you no longer receive messages from, and then click 'ignore user.'
4. If this person tries to message you, I believe they will get a message telling them that you cannot receive messages. I am not sure if it's obvious you have banned them or if it's a message similar to what you get if your pm function is turned off, but there is a message that appears.
Secondly, you can opt to not see forum members of your choosing when they post, whether it's threads that they start (in which case, you will not see anyone's post in the thread- it will be like it was never there) or their posts within a thread (in which case, you see the entire thread, except for their posts.)
1. Click 'dashboard' (on the taskbar next to 'profile.')
2. Scroll down to 'ignored users.'
3. Click on 'add users.'
4. Add the username of the person you no longer wish to see and click 'ignore user.'
5. Unlike the pm thing, users who you are ignoring in this way will not know you are doing this.
Both of these are reversible by navigating back and clicking undo/remove (or something to that effect) for whoever you are ignoring, if you so choose at some future point.
This option can help keep the peace if other means don't work. <img src="i/expressions/rose.gif" border="0">