Say we can ip ban them or something like that?
IP ban wouldn't work since its very easy to just change your IP or use a proxy from the start. Either way, the PM can only be sent once per 6 minutes so that does help with the rate limit. Creating account from a known proxy would also require payment.
Well in that case, why not totally disable the pms for newbie members? Only allow them to pm after they have reached the junior member rank? Bitcointalk.org is high on spammers list, i think i have received some spam in the past too, there needs to be a mechanism to prevent this.