I am still characterizing it, but noticed this option:
IP Check Date Limit
When checking for other users having used the same IP addresses as those logged for a spammer, this control limits the search to the last x days.
edit: I am not terribly sure it is working actually. I tried using the IP check on xf.com a while ago (B4?) and it timed out. In addition it seems to be timing out on my test forum which was a vb3.8 import.