Yes you can disable DHCP. But that won't help you because the intruder knows what the IP of your router. But if you assign static IP you may want to change the ip range to something totally different than the default. For example I set my router for the IP 126.96.36.199. I then gave all my servers and workstations static IP's. Then for wireless, I made my router give out only 2 DHCP IP's. I have a pic that shows what my computers with static are set to. ftp://sawatzky.dns05.com/ip.bmp
Use the router IP for the gateway and DNS server name. That should hopefully work for you.