I'm afraid I wasn't around to see it happen so It's hard for me to judge. He sounds a little trigger-happy with the ban-button...
... since when could Moderators ban anyway? I thought they could only suspend... O_o (I admin two invisionfree forums.)
I'll say give him another chance, but don't let him do anything without you knowing about it first.
(Aside maybe from deletion of spammy posts.)