|You invoke - enable - the Access Control List in the router. You go to the router's homepage (its website) and enable the list. You have to enter the MAC details for all devices permitted to use your wifi; and set the option "allow only these access"|
Also wise (in your situationby the sound of i...) to change the default password your router's management access to something else; and to note it carefully (somewhere secure) in-case you forget what you changed it to. I often email it to myself - suitably and vaguely identified.... Many also put on the base of the router - but based on previous posts here ("your brother's?) I wouldn't do that on this occasion.
And also use the password routine as in post 2. The more you tie the thing down, the more secure it is.
message edited by trvlr