It looks like the private message spammers have really started liking us lately. Almost daily I’m deleting spammers. For now the best is to “report as spam” as soon as you get a spam message: if more than a few people do this the person will be blocked from sending further messages immediately.
Other than this we are also looking into severely restricting the message-sending capabilities of people who have not done certain actions yet on the site. I won’t say which because this might be a hint for the spammers, but the idea is that if you haven’t done X or Y on the site you won’t be able to send more than X messages per minute/hour/day.
Sorry for the annoyance!