Agreed. Wouldn't be hard, not least because (unlike email) the site has a god's-eye view of every message a user is sending, so can see if a person is just send

Agreed. Wouldn't be hard, not least because (unlike email) the site has a god's-eye view of every message a user is sending, so can see if a person is just sending the same spammy message to everyone and penalise them accordingly. That, and minimum message lengths.