Is it possible to block the Wifi access for an individual application(Mac OS X 10.7)
Install and use LittleSnitch to easily block outgoing network connections on a per-application basis. (Can also configure incoming blocks as well, if you wish.)

If you're trying to defeat network copy protection in Adobe Photoshop and certain other apps -- you can prevent Photoshop from "phoning home" across the internet, but you can't prevent it from detecting multiple serial numbers on the LAN. At the LAN level it uses protocols other than TCP/IP to see if other same-serial-number copies of the program are running. LittleSnitch can't recognize or filter these non-TCP/IP communications.