The ability to remove Foundry users from the user manager interface will be coming soon in a new update.
They won’t cause any harm by existing, though I can understand how it makes assigning permissions a pain.
So until the update comes out, you are able to remove them through Foundry itself. You can do this by temporarily disabling Automatic User Management, launching the Foundry server, and launching the Inferno world, then navigating to Configure Users and removing them there.