This is something that in 15 years of Magento use we have never encountered. A developer, (whose extension we PURCHASED), will not stop sending marketing messages to the administration page of our Magento site.
He has found some way to break in to allow his commercial marketing messages and refuses to stop. Additionally, there is no way to delete them as they show up as a banner on the page rather than a message in the messages area.
Needless to say, there is no "unsubscribe" button either. Essentially, he is spamming our Magento 2.3 Administration site and we want it to stop.