How do I prevent someone from using the car?

No problem. Remove the coil wire and it won't start. It is the wire in the middle of the rotor on an older car. Do a google search for coil wire and coil so you know what to look for. It won't hurt anything and when you are ready to drive reconnect. Good Luck.

it depends on what make model and year car it is , personally I would take the car to a mechanic and have him put a toggle switch that opens or closes the citcuit for the starter. tell the mechanic you want a kill switch so the car can not be stolen, and he can hide the switch any where you and him figure out (put it somewhere you can reach from the drivers seat. Meanwhile disconnect a battery terminal get a wrench and loosen the terminal just enough to get it to come off. then when you go to drive you put it on real quick and twist it tight and drive on. Of course if she sees you under the hood she will figure it out.

change the ignition switch works good but is expensive.\

beat the pee out of the *****

park the car around the corner on a back street she is unlikely to find. I did this for months a friend of mines mom bought my roommate and me a car with the understanding I not let her daughter drive. I parked behind the apartment complex accross the rail road tracks. One day she looked over and saw the car and said "Hey that looks like the car? and took off the long way around the block. I hopped the fence jumped in the car and drove it back around to the front and parked it. when she got back it looked like I did not move, and she said well I guess not it was gone by the time I got there'

Just be out at friends when she needs the car.......

Take the car out first. go to the movies or something to waste time before you need to pick up your mom.

I think taking the wheels off would do it. Or removing the starter motor or similar.

Just disconnect one of the battery cables. Then reconnect when u need to drive.