There are a couple of ways to do this. The simplest would be to use a switch or router with VLAN (VirtualLAN) capability. This would allow you to assign wireless and wired clients to different VLANs which would prevent any communication between them. Unfortunately, consumer-priced products with VLAN capability still are not generally available.

If all you need is protection in one direction, i.e. keeping wireless users off the wired LAN, you can use the router-cascading technique described in our Setting up File and Printer sharing between two routers How To. Just make the wireless router the one connected to the Internet and the wired router WAN port connected to one of the wireless router's LAN ports.

If you want both wireless and wired users from reaching one another you can use three routers, as described in our One Internet connection—Two Private LANs How To. Only one of the routers, needs to be wireless, the others can be Ethernet-only.