Deleting an associated HABTM record

Ok, I'm probably being really dumb here, but I can't figure out how to delete an associated record when the association is HABTM.

I have the following:

Invoice:
   
    public $hasAndBelongsToMany = array(
        'Flag' => array(
            'counterCache' => true,
        )
    );

I can add flags to invoices no problem (just by creating $invoice['Flag'][...] and then saving the invoice), but I can't figure out how to delete them!

I've tried:
  $this->FlagInvoice->deleteAll( array( 'invoice_id' => $invoice_id, 'flag_id' => $flag_id ) );
and
  $this->FlagInvoice->deleteAll();

but I get "Call to a member function deleteAll() on a non-object" (presumably for the 'FlagInvoice').

If I add 'FlagInvoice' to the $uses variable at the top of the controller, I get the error that the table doesn't exist in the database.

I know I'm missing something really obvious here (I'm pretty certain I've even done this before!), but I've looked online, and I can't find an answer.  Can anyone help? Thanks!

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