Hmm I havent noticed a thing about the spam till now. Yet I think you can bumb it up to at least 100 posts. People who are out there to spam around won't hit the 100 posts before being not noticed by the staff and banned.
Aside of that, why would you need to PM if you have around 59 posts orso? Once you posted around 200-300 posts you notice that you might like some people because their attitude towards you, or they are always helpfull. I regulary PM Ollieb with problems, as I have seen him posting over 2 years on LO and 40ko. I dont think that spammers would give a darn about others on this board, and thus they won't take the effort to make more then 100 posts in order to spam others with PM's including weird photo's or the like.
Besides, 100 posts aint that much. 40ko is addictive, once you participate in several topics [serious topics about your army and the like] you quickly start hitting the 25/50/75 with ease. I used to post around 8 posts a day. Heck I think I post even more the 8 nowadays when I have the time.