Normally you can look on the side or bottom of the wireless router to find the default ip address and login info to access the setup for it. You should be able to find a way to restrict users by list such as the wireless mac filter access list or similar.
You can use this to find every computer that is currently using it via wifi and can choose to allow or prevent them from accessing. This is a normal recommended feature to prevent those you know that have been abusing your network such as neighbors constantly downloading things.
Such as Cisco routers default is 192.168.0.1 and Netgear is 192.168.1.0
but the router should have the exact it uses depending on brand.