I fixed this command admin please check this command is working or not if it is not working then please inform me
add this in onplayer death
[pawn]
if(pInfo[playerid][brb] == 1)
      {
          SetPlayerHealth(killerid,0);
          SendClientMessage(killerid,RED,"[Server] No Killing If Player Is Away");
      }
[/pawn]
add this under the enum piInfo....
[pawn]
new playerisbrb[MAX_PLAYERS];
new Float:Heal; [/pawn]
and just simply add this command in (!) command area....
[pawn]
else if (strcmp(cmd, "brb", true) == 0 )
   {
      if(!playerisbrb[playerid])
      {
       format( szMsg, 128, "%s Will Be Right Back",gPlayers[playerid]);
      SendClientMessageToAll(COLOR_BLUE, szMsg);
               GetPlayerHealth( playerid,Heal);
      SetPlayerHealth( playerid,100);
              TogglePlayerControllable(playerid,0);
      playerisbrb[playerid]=1;
      }
      return 1;
   }
   else if ( strcmp( cmd, "back", true ) == 0 )
   {
      if(!playerisbrb[playerid])
      {
          SendClientMessage(playerid, COLOR_WHITE, "You are not in the BRB 
mode.");
      }
      else
      {
       format( szMsg, 128, "%s Is Back On Game!" ,gPlayers[playerid]);
      SendClientMessageToAll(COLOR_BLUE, szMsg);
      SetPlayerHealth( playerid,Heal);
      TogglePlayerControllable(playerid,1);
      playerisbrb[playerid]=0;
      }
      return 1;
   } [/pawn]