I've seen this wrong all over the place, so I'm going to explain it.
1) Your script will probably have something that forwards or sends an IRCVERS. You need to alter this. The only IRCVERS you should send is IRCVERS IRC8 MSN-OCX!9.02.0310.2605.
2) You are now in IRC8 mode. You need to take a few other things into account: There is an extra character in the profile code next to the nickname. Profile codes are like FY, MX, G, etc. Subscribers are B and non-subscribers are O. An example nickname in the names reply would be H,U,FXB,.SomeNick, which means the user is Here, a User, and is a Female with no picture (X -- probably related to the X IE displays over missing images), and is a suBscriber.
This will prevent ANY false triggering of your script on the "No permissions to perform command" raw. This also will allow you to kick almost immediately after someone joins.
Someone should take these tips and write an addon from them. I'm not so great at mIRC (and I don't use MSN in the first place). Hope these tips help you people who still suffer from guest floods (unsubscribed guest floods, but guest floods nonetheless).
P.S. All MSN staff are unsubscribed guests. The MSN Chat Control apparently overrides the goggles (glasses) with the butterfly based on the H,A/S/G,GO, and not the ' (however it seems to act weird without both of them -- perhaps this is to fix something that's flawed in their server?).