if you use the /ialmark idea the parting nick will be deleted from the internal address list if that channel is the last common channel with the bot, so the mark you make on it will also be deleted. Using the ial and not using /auser or /ruser means you dont have to worry about quits or parts, Khaled has done it all for you. Not much point either in using user levels for this as when they try to use a command you will have to check if the channel name is in their $ial(nick).mark if not then they dont have access to use that command.
