Sending, receiving, and organizing email in Outlook.com
I initially set the flow up to move these ****ing annoying emails to the Deleted folder (to make absolutely sure it wasn't deleting everything in the Junk folder). That worked fine (I was still getting the odd bit of spam from elsewhere that ended up in Junk) and now I have amended the flow to delete the gostarmedia emails I can see them appear briefly (a few seconds) in Junk before they get removed automatically. None have slipped through yet so double check you've got it all set up correctly.
Totally agree with you about blocking the whole domain - it can't be that difficult to make a change to Outlook so that we can do that!