[RajatPawar]

You can do something like -

pawn Код:

new someSkin = 12; // A skin you want to hide

public OnPlayerRequestClass(playerid,classid)
{
    if(classid == someSkin)
    {
        SetPlayerSkin(playerid, classid + 1); // Skip that classID
    }
    return 1;
}

Untested, but COULD work.