“Blocking” users (or entire instances) from the Akkoma front end only hides the posts from you, even if you’re an admin. It’s sorta like muting them.
Non-logged in visitors to your Akkoma site can still see activites from these users. And other people don’t even know you’ve blocked them.
To really get rid of them, go to the Admin-FE.
The first page has individual users, here you can deactivate them.
And for instances, you can go to Settings, then to MRF, scroll down to Simple MRF and under “reject” you can add the bad instances.