Parents use my car without asking?

First do not leave the keys around.

Second fit a second immobiliser.

Third fit a tracking device.

Leave the car elsewhere so they don't know where it is and therefore cannot use it.

You should move.

If you are over 18 and listed as the only legal owner on the title then YOU decide who drives the car and when. Of course, they might decide that you need to pay for your own food or place to live etc., but few decisions come without consequences.

If you are under 18 years old then you cannot be the only listed owner on a car title. You must have someone over 18 listed as an owner, and in most cases that means a parent or parents are listed as the legal owners. And if they're listed on the title as owners, they have full legal rights to drive the car whenever the heck they want and it makes no difference whatsoever who actually paid for the car.

Then again, nobody can drive a car if the car doesn't start... Hypothetically speaking, if you were to pull the fuse for the fuel pump as the engine was running, put that fuse in your pocket and then let the engine run until it stops running from a lack of fuel... then nobody could drive the car because it wouldn't start again until the fuel pump fuse was reinstalled. If nothing else, that might keep anyone from driving it for a while. Or maybe you just "fix it" every time you need it and "un-fix it" each time you park it until your parents get fed up with trying to use your "unreliable piece of junk" car and they start driving their own car(s) again.

put a security system on it so it kills the ignition so they can't start it weather you let them get down the street then let the security system turn the fuel pump off so they have to walk home or the ole secret switch under the dash is up to you.

Take away their keys.

If your name is on the title then you can call the cops, if that's something you really want to do to you parents. If their name is on the registration and title, then you have no choice. Technically it would be their car.

As a parent I do my best to return the disrespect to my kids. It's fun sometimes. Not with a car though. I like waiting till they have something yummy and eating almost every bit of it and then put it back empty. I make sure to make a mess and leave it when I visit at my kid's house. It's a nice hobby. Maybe they're doing on purpose, maybe you are getting punked. Unfortunately they paid for everything they have and you consumed; you don't have much of an argument.

dude they are your parents let them.

ask a dealer how much it costs to change the ignition key.