With peerguardian, if you can find the IP in the log window (you can increase the number of lines seen, mine is set to 100, under the
settings tab) If you can find the IP place mouse pointer over it and left click. Once the line has been selected, right click and a menu will appear. This give you three options block for 15 mins, block for 1 hour or block permanently - choose the one best for your purpose.
This should add the IP to your blocklist. Please note: This will only be blocked by PeerGuardian if it is running in your system tray, so allow Peerguardian to run on startup (if you haven't already). Peerguardian forums available here:
http://forums.phoenixlabs.org/
BTW the blocklist.org website is down at the moment, hence updates are unavailable for peerguardian - I have been promised this is being looked into, and will be sorted out ASAP