if the user changue nick, u lost everything. make with gate/address
($address or $ial)
in irc servers that's works fine with only nickname, but some servers dont use your hostname or gate "nick!randgate@bla.server and make a random gate or something when you register.
And u can use the ignore identify (see help for usage)
;- if ( !$ignore( gate here.. or nick or something ) ) { .. }
xD