Turn on new member moderation.
The disadvantage is that turning off moderation becomes a task for
the group owner or managers.
You mean turning *on* moderation.
I have had to do this on a Google Groups list I maintain. But mine is
low traffic with few members so it made sense for me to do that.
No, turning on message moderation for any new member is a one-time admin task.
But if the normal operation of the list should be unmoderated there
is then an additional admin task of reviewing new members (presumably
by looking at their first message) and either deleting them or
un-moderating them.
For lists with infrequent membership changes this is easy.
On lists I manage (very infrequent membership changes) I haven't even
had to look at the messages -- just the construct of the email name
is enough to know they are constructed by and some spam program.