[grand.Theft.Otto]

pawn Код:

public OnPlayerConnect(playerid)
{        
    if(!strcmp(Name,"[ABC]", true) == 0)
    {
           // Code here
               SendClientMessage(playerid,COLOR_BRIGHTRED,"You Must Have A Tag Contained At The Beginning Of Your Name. Please Change It Blabla. You Will Be Kicked.");
           return Kick(playerid);
    }
}

I guess you could give that a try, it's something similar to what you need but you need to edit it a bit more.