Hi
Closed lists will just be more efficient. If the bounce message is clear then most genuine contributors will get that and respond appropriately.
I've only dealt with one real, actual problem as a moderator and it was purely accidental that the offender wasn't a subscriber and I was able to catch it. We should expect some spammers to join but even this hurdle will eliminate the majority and we can boot any subscribed spammers quite quickly.
ATB
Stevie