There are scenario’s you can disable that will get rid of some of these but for the most part these are ‘Car Generators’ that are included inside of the vanilla ymap files that make up the world. You can explore with CodeWalker to see all these locations and which ymaps they are contained in.
There are a few natives for vehicle generators that I’ve never tested before myself but you could play around with them and see if they actually help solve your problem. I went the other route with this problem and just prevented players from opening any parked vehicles without first running through a car jacking script to have control over it.
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RemoveVehiclesFromGeneratorsInArea()
https://runtime.fivem.net/doc/reference.html#_0x46A1E1A299EC4BBA -
SetAllLowPriorityVehicleGeneratorsActive()
https://runtime.fivem.net/doc/reference.html#_0x608207E7A8FB787C -
SetAllVehicleGeneratorsActiveInArea()
https://runtime.fivem.net/doc/reference.html#_0xC12321827687FE4D