There are certain conditions we have in place to prevent PM spammers, of which we get qute a few. Unfortunatly that results in the occasional false positive, but its a large enough problem that the solution we impolemented is necessary, and the conditions to trigger the spam filter are fairly unique, though I am not going to go into what those actually are.
You should be able to PM now. please let us know if you are still being blocked.
Administrator Kaolian