Emails deleted in GMail, even from the trash folder, will then sit in the Bin folder. Is getting rid of them from here what you mean by 'delete forever'?
Not sure there's a way to do this from a client app (I know neither GMail or K9 do this afaik) you have to go to the web site.
That said, they are doing no harm there. GMail gives you a huge amount of server space and they are not synced with your phone so take up no memory or data.