That's no problem at all. PPC is really flexible when it comes to who you want to see your Pay Per Click ads and who you don't. National PPC campaigns are great if you are an online retailer that ships nationwide, but not so great if you're an estate agent that focuses on a couple of towns within a region.
The Google AdWords system allows advertisers to easily target anywhere in the world. It can be an individual town or a few towns miles apart. So, lets take an estate agents as an example. They only sell and let houses within a 20mile radius of Telford, Shropshire. That's not a problem for the AdWords system.
It works on the IP address of the users computer and what they search for. So someone in Telford would search simply for "estate agents" and the ad would show. But someone in Manchester performs the same search and the ad won't get shown. If the keyword in the campaign is "telford estate agents" then the ad would get shown if the person was in the radius area, or in Manchester. The Google AdWords system understands that although that person isn't in the radius set, the ad should still be shown because that person is looking for something local to that area.
Geo targeting is a great way to limit who sees your ads and obviously means your budget won't go down as quickly. It does mean though that the number of clicks your website gets will be vastly reduced.
This is something we understand at PaulSearch, and is one of the questions we ask when we set up new campaigns. Once set, the towns, cities, or radius selected can be changed at any time. That's not a problem. The great thing about PPC is that you can change pretty much anything you like, as often as you like! Pay Per Click is quite a complex thing, which is why here at PaulSearch we take that hassle away from you!