I don’t think there’s a setting to remove the ‘delete and block’ button, but you should be able to hide it with a little CSS in a custom theme component.
It is quite a useful button though, especially if you only delete spammers. Often they’ll just re-signup if the email and IP aren’t blocked.
I’m afraid there still isn’t a way to manually add an email to the blocked list in the UI (There is a workaround using the rails console, How do I manually add an email address to the block list?, but that’s not particularly useful for day to day use). I did think there was a Feature request for it but I can’t seem to find it. 