// at the top
new GlobalChat;
forward DisableGlobalChat();
forward EnableGlobalChat();

// somewhere not inside a callback or function
public DisableGlobalChat()
{
      GlobalChat = 0;
}

// somewhere not inside a function
public EnableGlobalChat()
{
      GlobalChat = 0;
}

public OnPlayerText(playerid,text[])
{
        return 0;
}

new bool:global = false; // before GameModeInit / FilterScriptInit

if(!strcmp(cmdtext, "/ed", true)) // Enable / Disable
{ 
      if(global == false)
      {
           global = true;
      }
      else
      {
           global = false;
      }
      return 1;
}

public OnPlayerText(playerid,text[])
{
        if(global == true) return 0;
        return 1;
}