I just had this problem yesterday. A guest logged on for longer than usual doing four different things. Viewing the forum index, private msg, user profile, and who's online. I banned the IP address to stop it. When I unbanned the IP the guest came right back. I banned the IP again for a longer period and it's now gone.
I checked my web logs and it generated about 1000 requests in 16 hours or about 20% of the requests my site usually gets in one day.
Turns out it is a webcrawler from www.turnitin.com
. I have no freaking idea why they're crawling my forum. They didn't appear to be interested in any non-forum stuff.
Looks like they've been visiting my site for the last three weeks.
Hopes this helps others with this problem. BTW my site is http://www.cavycam.com