My suggestion is simple. Deny the use of /shout, /yell, and /tell for characters younger than a certain age, and make the free trial for new accounts last exactly that long, so that free trials can't be used for spamming.

For example: You would need to be more than three days in on your character to use /shout, /yell, or /tell, but your trial runs out after three days. This puts a pay gate on their account that, if they paid, it would be unlikely they would get a return before they were banned for RMT spam.

This also wouldn't inconvenience new players, since most content in the first few days is solo and tutorial.

I feel this would greatly reduce the amount of RMT spam we get outside of compromised accounts.