If you use mirc's in-built aop feature, then it's a lot easier to exclude them from being greeted. Just change the join and part event to this:
on !*:join:%greetchan:if (%greet) && (!$aop($fulladdress)) .notice $nick 7 $nick Welcome to %greetchan 11 Have a nice 7 Chat
on !*:part:%greetchan:if (%greet) && (!$aop($fulladdress)) .notice $nick 7 $nick You left from %greetchan 11 cu 7 later
This will match any address or nick within the fulladdress in the aop list to exempt.