[AtItsMax]

pawn Код:

new PickedClass[MAX_PLAYERS];
public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
    if(PickedClass != AGENT && GetVehicleModel(vehicleid) == hydra id here)
    {
        ClearAnimations(playerid);
        SendClientMessage(playerid, -1, "[*]: You should be Agent in order to drive hydra!");
    }
return 1;
}

This is an example, you can do other's this way.