[Deduction]

Actually,
Preventing this would require a small script inside your code.
Possible ways to do this:
1.
lolumadd_ the way your explaining it is that they go into a helicopter and spawn vehicles all at once.
A way to avoid this could be:
Create an X, Y, Z Variable for every Vehicle ingame like so:

pawn Код:

new float:VX[MAX_VEHICLES];
new float:VY[MAX_VEHICLES];
new float:VZ[MAX_VEHICLES];

Then you would need to OnPlayerExitVehicle

pawn Код:

new float:X, float:Y, float:Z;
GetVehiclePos(vehicleid, X, Y, Z);
VX[vehicleid] = X;
VX[vehicleid] = Y;
VX[vehicleid] = Z;

Then you would need to create a timer that checks all the Vehicles.
Checks whether theres a player in a vehicle and if not, checks the vehicles position and if its not what the vehicles positions been set to, then it would set the vehicles position to its set coordinates.
Preventing Mass Vehicle spawning/teleporting
2.
[Fix for [MM]IKKE Idea of how it works]
Create VARS for a player which is Set to 1 when they enter a vehicle.
A Timer is created for say atleast 1 Second. If the player gets out of the vehicle when the timer is running,
Send a warning message to admins saying possible Vehicle Hacker.
( This could be a bad way to detect it )
[Another Fix]
3. Using a Plugin called MapAndreas, you could find the Z coordinates in the location of where solid ground is and if a car is more than +10 of the Z Coordinates then Send a message to an Administrator alerting possible airbreak or vehicle hack. Though Exceptions would apply for Aircraft.

Its just a few ways that could help.
But its not impossible to detect them