Relays could really cut down on the traffic going trough the relay and to the clients if they lock out spammers. No need to feed all that traffic to clients.
Relay operators should get to decide who the spammers are? Hm. My knee-jerk reaction is that's a bad idea. But, then again, as 'tenants' of a sort, I suppose they have that right. Then blooms a relay market for who censors whom/what, and users (customers) make their choices from there. I guess I see both sides of the argument...