Just disable DHCP and set the thing up in AP mode... if you dont want to use the "router" then just dont use it. If you want it tighter then use MAC filtering so only you connect to it.
I don't want the router to do any routing at all. How would I disable it? I just want it to be an Access Point and I will give it a static IP on the subnet I want it on.
disable the firewall (if there is another firewall before it)
give it an ip address ie: other than 192.168.1.1 if that is being use by another router.
Disable DHCP (if there is something else giving out ip's)
Plug the cat5 cable to the LAN port. (yes LAN not WAN)
Then secure your wireless if you wish
I have setup many clients this way to expand their wireless coverage in their homes as long as they have a wired backbone in place.
P.S. MAC filtering does nothing for blocking unwanted guest using WEP or non-secure, it just slows down a would-be hacker.
