I do not know this router, but there are 2 basic ways you can ensure no one is using your network. the first is allowing only computers with specific mac address to enter the network.  To find out a computer's mac address, Type mac /s %ip address% in the command prompt. The second way is encrypting the network with a wep key. Read more about these 2 ways in the router's manual which is probably assigned on a CD you got with the router. If you need more help, post again.