@afewbugs is he maybe just hitting move to spam? There's a good chance that would block you; tho there is a block sender item in the 'more' menu.
I'd also wonder if there is a keyboard shortcut for that.
I finally managed to persuade my dad off that platform a couple of years back after he got hacked badly (a script that emailed everyone in his address book asking for help, redirecting replies to some other address, and deleting all his contacts) - so can't directly check any more. But the spam one seems most likely. He'd delete mail to keep his inbox empty, and if you select mails to delete, the spam button is the X right next to the bin; I found mails he'd moved to spam a few times