Can you set an 'allowed range' of IP addresses?
If so, set the router to accept (for eaxample) 192.168.0.50 to 192.168.0.60 (submask 255.255.255.0)as an allowed range, and get them to change to an IP address in that range.
Of course, this will block your other PCs from the LAN and the Web, but will also block them from accessing your LAN. You then only need to change the IP address of one PC on your LAN to join the party.
It's much quicker and simpler to set an explicit IP address on a PC, than to set up a whole host of security settings on each PC.
My Netgear router's 'allowable range' setting is under LAN setup.