agreed, you should set a wep (128 bit ideally), or better still use wpa.You should seriously consider configuring WEP and set up a Key Code for security.
you could use the admin pages of the router, but that not guaranteed to work, especially if the unwanted user is not using your dchpis there a way to find out who is on my linksys wireless network?
damn so annoying when people change there avatars, can never tell who it is - only just noticed it was you geoffhe never said anything about people being on there without his consent