Jim,
I did not want to mark it as spam even though it is, because I would
not want to get bumped from my own list for marking a message as spam.
So far at least you're probably safe with Gmail. I don't fret much when an +owner message lands in my Spam folder there, if it actually is a spam message, though I admit I haven't marked any as spam either.
What avenues are available to put an end to the spam to the owner
account?
You can change your subscription so that you aren't sent +owner messages from non-members, but in some cases that kind of defeats the purpose of the address. I usually recommend that at least one active moderator in the group accept messages from anyone.
Given that you use Thunderbird (as do I) it is probably safe to mark them as Spam in its interface. But with IMAP I'm not totally sure that doesn't get reported up to the server. It may depend on what you tell Thunderbird to do with it. If you have Thunderbird move it to a Junk folder in Local Folders that's probably invisible to Gmail, but having it moved to Spam in your server account might be equivalent to marking it in the Gmail web interface.
Shal
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