A few months ago I got an error message when I tried to post to the forum. It said that I was on a Spamhaus blacklist of know spamming IP addresses and have been prevented from posting. This is happening to a number of members now, so here's some explanation.
The forum uses a system to stop known bad IPs from posting. It links to blacklist maintained by a group somewhere that lists IP addresses for known virus infected botnets and spam farms that often post messages that we wouldn't want. Generally this is The Spamhaus Project.
This has happened to me too. For me it was after a power failure where my DSL modem grabbed a fresh IP address from Verizon that has previously belonged to an infected machine. To remedy this, you should go to the black-listing site and request to be removed, after you have made sure that you don't have a virus or trojan on your computer. The error message should include the specific blacklist being used, and guidance to remove yourself from it.
I've looked into this before, and general consensus in the board community is that turning this feature off can lead to some very bad things, so I'm inclined to leave it alone. The majority of the time that you get this message it means that your computer is infected, or someone behind your router is infected. You should have that looked at. If you get request a delisting and then get blacklisted again, there is definitely something wrong with your computer, or the computers of those near you.