Is spam by PM really that much of a problem? I've moderated a very busy AV forum previously & although the amount of spam posted is undoubtedly a big problem, can't say that I ever saw much by PM.
At one time 12,000 porn spam PM messages in about 24 hours. That's a big problem.
Generally if I see a new member who is obviously not a spammer needing pm access I can switch it, since this topic started a bit over an hour ago while I was still asleep it didn't get seen.