[Deal-or-die]

Alright you could try doing something like this.

Example:

pawn Код:

public OnPlayerRequestClass(playerid,classid)
{
    if(classid == 3 && PlayerInfo[playerid][pwhatever] == 1)
    {
        SendClientMessage(playerid,COLOR_GREEN,"This class is unavailable to you.");
        return 0;
    }
    return 1;
}

Also have a look at this, it may help
https://sampwiki.blast.hk/wiki/OnPlayerRequestClass