Yes it is possible, just set your vehicle dimensions high enough. Whether 30 or 60 FPS, it won't budge (I trialed with around 20 m each axis, 99% unmovable so I'm guessing 30 m onwards should do the trick 100%).
Is it possible to make car unmoveable/unrotable by Ped/player pushing via vehicle handling? (with no scripts)
The reason why a mass change wouldn't work is because pressure is also a factor to work by. To keep it simple, basically when you apply a force to an object, the object won't move unless that force is being applied to a considerable amount of the object's surface area, say at least 20% for argument's sake.
With normal vehicle sizes, the player's surface area is lower but not exponentially lower than the vehicle's external surface area. It's probably in that 20% mark if not slightly more. So with enough force, no matter the mass, it will move.
In contrast, if there's an exponential difference in the surface area of the force being applied against the entire surface area of the exterior of the vehicle, that's when it's impossible to move. That's essentially why needles and knifes pierce through objects rather than move them. The force is great but its acting on just a small point, so therefore that point will rupture but the entirety of the object remains still. That's why higher dimension stats of the vehicle is the way to go.
Once you do that however, you'll want to use my trial ratios I gave previously to get the vehicle back to practical speeds when driving:
- 4 : 3 or 1.333 : 1 - acceleration : dimension
- 1.465 : 1 - top speed : dimension