[KennethRandall]

Why are you using a table (more space) if you got only 1 function in it?
Try

pawn Code:

new Cars[MAX_VEHICLES];
new Skins[MAX_PLAYERS];
new Job[MAX_PLAYERS];

Add someone in OnPlayerCommandText

pawn Code:

if(!strcmp(cmdtext, "/police", true))
{
  Job[playerid] = 1;
   return 1;
}

if(!strcmp(cmdtext, "/civilian", true))
{
  Job[playerid] = 0;
  return 1;
}

Remove the Cars[Police] =, and add also at your OnPlayerEnterVehicle

pawn Code:

public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
  if(GetVehicleModel(vehicleid))
 {
    switch(GetVehicleModel))
   {
      case 596, 598, 597:
      {
        if(Job[playerid] != 1) return RemovePlayerFromVehicle(vehicleid); // kicks them when they'r not job 1 == police
        return 1;
      }    
    }
 }
 return 1;
}