One solution is to do the lookup manually, but its ugly and requires relatively more complex code to monitor the RAW replies and such.
The best solution would be to redesign the way you store usernames to never have to do lookups on the user input ($2). This would mean if your user input is an address mask, that is the mask you put in your hash table, exactly as given. It makes the system less flexible (the user has to remove the user with the exact same mask they added them as) but its a lot simpler to deal with. It's also how most eggdrop bots work, as well as mIRC's userlevel system. It's a proven system, and there are other more technical reasons the design is as such, but I'll leave that as a thinking exercise for you.
