Could it be that they're using the same IP as someone who did run a bot on the site also?
No. It's difficult to ban IP addresses on that basis as bots usually use proxies so trying to ban them usually results in a lot of false positives. We have experimented with it recently but we've ended up implementing some more complex anti-bot measures instead. That's work in progress at the moment and we've got more to implement soon.
Edit.
Have a laugh at this dudes post history.
Interested in a multi-threaded bot for this site? At the moment, the bot can easily work with 1000 accounts at the same time, the average time between the climes of all accounts is 60-70 minutes. The price of $ 600 to anyone interested in writing in ps