If the postman drives onto the same bit you park your car on I can't see what you can do. Could you supply a photo so we can see the problem?

The only way to stop the gravel from scattering is to bed it into cement. In effect you are turning the gravel into concrete but without the smooth finish. The easiest way to do it is to rake up some of the gravel, spread DRY cement over the area and water it in with a fine spray from hosepipe. Spread more DRY cement, water that in - doing it in layers makes sure that the cement goes down through the gravel. Let that set for 24 hours. Replace the gravel that you raked up, spread a thin layer of DRY cement and water it in. Leave it to go off for a couple of hours to check that the top gravel is set without cement being visible - add very thin layers of cement or gravel as necessary.
Don't water in the top layer of cement if rain is forecast - let the rain do the job instead.
And don't just put cement on the top as weight of the first car will just cause it to crack.

Although you can't put a fence could you make a definite divide with a row of low growing plants such as lavender - not quite a hedge but enough to make the postman think twice about driving over.

the trouble with cementing the stones is you then have an impermeable surface - I don't know if there's any legislation in France regarding this, but there certainly is in the UK now, all hard surfaces in front gardens must be permeable to a degree.

What you really need is self binding gravel - these are gravels with a high level of dust in them, they're spread and then rolled or hit with a whacker plate (impacter) to make a fairly firm surface which is still permeable. Over time, if vehicles constantly drive over them, the surface may actually become hard and solid, and can then be broken up with a fork, relaid and re rolled, but it doesn't sound as if you have a high traffic area. Two examples of this type of 'gravel' or stone used in the UK are Cedec and Breedon Gravel (this one's used at Buckingham Palace). There may be suppliers in France who have their own version of these.

There's also resin bonded gravel, and there is a permeable version of that too - it might be possible to get someone to come out and resin bond your existing gravel, I'm not sure, but might be worth checking out.

The only other option I can think of is to create a paved sort of 'path' within the gravel and ask the postman to make sure he keeps the wheels of his vehicle on that paving - but the paving will need to be on aggregate and a concrete mix to take the weight of a vehicle.