That has nothing to do with mIRC's address book. The closest thing you can do is set yourself +i (in 6.35, Alt-O > Connect -- in 7.x, Alt-O > Connect > Options > Use Invisible Mode). However, all that does not prevent people from seeing you in channels or doing a /whois on you to see if you're online or simply trying to /msg or /notice you to see if you're online. In the end, it really doesn't do much for what you're asking, but it's the closest thing there is. Basically, if you don't want someone to know you're on, use a different nick.