If unauthorized people are using your wifi i'd think it's unlikely theyd join same irc channel as you but it sounds like you have a unique situation.
ON *:JOIN:#:{
  if ($gettok($address($nick,2),2,64) == $gettok($address($me,2),2,64)) {
    if (mynick !isin $nick) || ($gettok($gettok($address($nick,5),2,33),1,64) != $gettok($gettok($address($me,5),2,33),1,64)) { ban -k # $nick 1 Unauthorized clone. }

Should do what your asking just change the "mynick" thing to the string that you require in your nickname. If a user joins with your ip that doesnt share same ident or have that string in nickname they will be kick banned.