Okay, time to shed some light over this. First and foremost; its a fake, no one is hacking your account unless you did something idiotic like give out your info to a friend or whatnot -- but from what I have learned there is a Silkroad clientless program, an injector program I think called SR33 (unverified) if you will that gets a list of a majority of the people in your server and allows the user to simultaneously PM them with whatever they want. This sort of thing happened in the Odin server about a month ago, and from what I could gather, it was more than one person doing it. I know this information because one of those PM spambots happened to reply back to me and after a little persuasion, he/she/it told me exactly what he was doing and how.
And if youre having trouble blocking them, refer to this thread below. Posted when this problem started arising in the Odin server. Have a nice day.
viewtopic.php?f=50&t=93289