hozz létre egy tömböt ( 
max_players ) és ha 1-en van azt csinálja ha 0-án return 0; 

kis logika..
[pawn]new Boost[MAX_PLAYERS];[/pawn]
[pawn]CMD:booston(playerid,params[])
{
    Boost[playerid] = 1;
    return 1;
}
CMD:boostoff(playerid,params[])
{
    Boost[playerid] = 0;
    return 1;
}[/pawn]
[pawn]if(PRESSED(KEY_FIRE))
{
    if(Boost[playerid] = 1)
    {
    GetVehicleVelocity(GetPlayerVehicleID(playerid), X, Y, Z);
    SetVehicleVelocity(GetPlayerVehicleID(playerid), X * 1.5, Y * 1.5, Z * 1.5);
    }
}
[/pawn]