One solution that worked well on a game I played long ago (can't recall atm) the solution to spamming was, in addition to a timer (no more than 1 /tell /msg every 3-5 seconds), and the blacklisting was community driven.

so if a character was blacklisted by players for sending unwanted tells, once that player reached a threshold set against the server population ratio, that player was automatically muted from /tells /shouts /msg for a period of 8, 12 or 24 hours (based on the number of blacklists submitted)

And it was tied to the account, if their alts performed the same actions and received another blacklist mute, then all the characters tied to that account were muted for that cumulative period of time.

This seemed to have an eventual effect where the gold sellers were quickly running out of alts to spam with and forcing them to wait to advertise, or purchase another copy of the game, which soon made it UNPROFITABLE, and they eventually faded away.