4164,
First off, great scanner to own...
Now on to your question...I am not sure what you're asking. You can't "lock out" just the sites; the radio needs some way of hearing a control channel to do the magic that it does to trunktrack.
Let me see if I could explain this a little better...
Step one: Select a site you wish to monitor (i.e.: Beacon)
Step two: Create a system in the 996 and program the Beacon site's particulars (LCN, quick keys, etc.) into it.
Step three: add all the talkgroups you wish to monitor on the system you created in Step 2, creating related groups - up to ten - as you go along (i.e Countywide LE TGs, Local LE TGs, Fire TGs, Gov Svcs TGs, etc.)
Repeat the process for the next site you wish to monitor, and the next, and the next.
Orange is served by five sites (Arden, Beacon, Ellenville, Graham, and Schunnemunk), so, in theory, you should have five systems created - one for each site - each with their own assigned system number in your scanner. They each would need their own scan list (which, in my case, is programmed identically in each system - thank god for cut-and-paste). Now you can select and unselect the sites, using the appropriate system's assigned quick key(s), as you travel into and out of the ranges of each system - or as your needs otherwise desire.
I hope this helps,
SCANdal