Well that depends. If these counties are in the same state and use nothing but conventional freqs, that would work. However if you need to listen to their trunked systems, you need to do a bit more programming.
You might not even need to manually lock the undesired county out - you could use a GPS and a function known as Location Based Scanning to do that for you
The Easier to Read manual is certainly a place to go - but this article from our wiki (links are always blue) contains links to just about everything you need to get started. There's a very detailed article on Location Based Scanning linked in our DMA FAQ....
Program your DMA Scanner - The RadioReference Wiki