new dead[MAX_PLAYERS];

public OnPlayerDeath(playerid, killerid, reason)
{
   dead[playerid]=1;
   return 1;
}

public OnPlayerCommandText(playerid, cmdtext[])
{
   if(dead[playerid] != 1) 
   {
     SendClientMessage(playerid, 0xFF0000, "You must die to use commands.");
   }
   return 0;
}