BTW, if you run bitcoin-qt over Tor, I recommend decreasing maxconnections to 3 or 4. It takes a while to build all the connections because (apparently) a lot of bitcoin nodes don't accept them. I don't think bitcoin-qt can be configured to block specific IP addresses, but there may be firewalls or ISPs interfering with connections from exit nodes. The other concern is that you may not want to run bitcoiin-qt as a hidden service since there are known attacks to deanonymize hidden services.