I personally wouldn't show the customers a clear limit of your service area. To define that limit in your own mind makes sense, but to turn them away before they come could be risky. At some point, that ultra super high dollar long term contract could present itself. It is my belief that its better to answer calls and say its too far, then to entirely shut out those phone calls. You may hit hard times at some point and need to take on every job you can find. While your market seems vibrant and booming, all businesses at some point hit a tough spot. As your overhead rises, the damage that hard times can inflict also rises too. My advice is to absolutely not limit your phone calls. Limit which calls you are willing to bid, but never limit the calls. None of us have a crystal ball. You never know what might come down the line.
For example, I met a guy recently and got to shooting the breeze with him. We had nothing in common. The conversation was light and fluffy at best and I was eager to get back to what I was doing. I almost cut him off politely and got back to my task at hand. I was actually in his house helping a buddy install baseboard heaters(buddy was desperate for help on a Saturday and I owed him a favor or two). This guy asks me about what I do for a living. I told him. He then tells me he is in charge or operations at Sandvik nearby where I live. Tells me flat out he is very impressed by how hard I work and my professional demeanor while working in his home. Tells me they spend 80K a year on pruning, fall clean up, spring clean up, and hedge trimming. Then tells me he would like me to come to the plant and sit down with him and look over the numbers and if I can match the numbers or stay close to them(above or below), he would like me to take over and replace the current contractor. 80 grand a year from one account alone is a big deal to me. For some it isn't. In my market, it is. Especially for low liability, low overhead work.
My point is that I nearly closed a door and blew this guy off. Little did I know, a door was about to open rather wide for me. I encourage anyone not to close doors. Doesn't mean you have to go back and forth through those doors, but avoid closing them. It may also be wise to find a top shelf tree company in those regions that you are trying to avoid and refer the work to them. I don't know what its like in your market, but in my service area, there are a handful of us that are the main names that gobble up the bulk of the work. We kick each other work constantly. I kick so and so work because I hear they are a little slow, or because they can do a better job of it from their bucket truck. In turn, I get kicked a lot of referrals because some of those guys aren't interested in bigger trees that their buckets cant get high enough into, or they are too overwhelmed to take on more work. We rub each others back and often team up on jobs. Also, when one tree company suggests another, that's a big statement being made to the customer. They tend to have a lot of faith when one company says another is very good and to call them. So referring out work outside of your turf, might help generate even more work referred to you inside of your desired service area. I don't know for sure. It might be more competitive where you are. Here, we are pretty relaxed and amongst the group of established guys that dominate my service area, we hook each other up like crazy. Especially with asshole customers and jerks looking for bids, We have a way of making it tough for them to get a local bid.