It appears that Primes has been targetted by an automated spambot that is configured for the particular version of Drupal that we are running. The messages are coming from unique IP addresses distributed all over the world, so it's most likely the result of a trojan application running quietly on people's machines, unbeknownst to the individual users. So unfortunately, we cannot just block an IP address to stop the problem.
The timing (over the Christmas holidays) is particularly unfortunate, as our staff resources are minimal. This is a top priority for us, however, and we will be looking at a fix starting on Monday, Jan 5. In the meantime, we will be doing our best to keep on top of the problem, but with New Year's day tomorrow, followed by the weekend, it's likely that the messages will continue to build up over that time.
The new version of Primes that we are working on will have measures to prevent this type of thing from happening.