How do you politely refuse other unnecessary things you are told to fix, when you take your car for a simple job to a car mechanic?
You ask them to do exactly what you want. I suggest you listen as what they have to say may be good advise.
However, do remember that if what they advise you is safety related and they consider your car too dangerous, they can, and will, contact the police who have the power to take your car off the road. The cost of this will be at your expense...
Short answer, no shop should ever do work without first explaining the costs asking for your consent to do it.
Long answer: I encourage individuals to understand as much as they can about their vehicle and the problem it has before they go in.
Most mainstream tire/alignment/brake service shops aren't there to screw you. If you walk in there with a deer-in-the-headlights look, they'll take it upon themselves to make sure your vehicle is safe, because if something fails right after you had it in their shop it could end up on them.
As far as alignment goes a good shop will inform you of worn tie rods, drag links, etc... and if new parts are required, so is an alignment. These are all things that are good for your rig, and should be done. If an alignment is needed, and you don't do it, 1 - It isn't safe, and 2 - You'll have premature tire wear (tires cost way more than an alignment).
Brake service is also something that everyone gets mad about. "...but I didn't want new rotors..." - I hear this a lot from my friends. Brake rotors must always be surfaced with new pads - ALWAYS. Sure, plenty of cheapo home mechanics just throw new pads on, but that isn't how it should be done. The tech will check the spec, and if the rotors will be out of spec after surfacing, they'll save you the surfacing charge and just put in new ones. These days, new calipers are also required almost every brake job on modern cars.
I'm not saying there aren't shops out there that will screw you, but most of them probably aren't. Proper maintenance of all aspects of a vehicle makes the whole thing last longer, so plan on having money to put into it especially if you like to save money by buying used vehicles (like I do). For most cars and alignment is 40-80 bucks and worth every penny if you need it.