The flag way should work, Here's code I use in one of my projects to acheive what you're trying to.

class Vendor_Modulename_Model_Observer
{

    public $eventComplete = false;

    function sales_quote_collect_totals_after()
    {

        if($this->eventComplete){return;}

        $this->eventComplete = true;

       // put module code below here

     }

}

The flag way should work, Here's code I use in one of my projects to achieve what you're trying to.

class Vendor_Modulename_Model_Observer
{

    public $eventComplete = false;

    function sales_quote_collect_totals_after()
    {

        if($this->eventComplete){return;}

        $this->eventComplete = true;

       // put module code below here

     }

}