this suggestion has been posted several times. This can be abused ridiculously and thus will never be implemented. You can for instance block people you know will raise the red flag on your selling unperms for instance, or designating you as someone who has welched/scammed before in the bnp.
However, pm a mod if you have problems with certain user(s) spamming your topics/harrassing you on the forums. Use the block function to block pms.