Excellent list, with some sound and sensible solutions.
I’d add another one, which is arguably worse than all of these put together - though luckily, quite rare.
Unsubscribe people straight away. (Or at least as soon as possible; perhaps there’s an email already queued up ready to send in the next couple of hours to the user in question - fair enough). However, I’ve had email lists tell me that it’ll take 7, even 10 days to process the request. I can’t think of any possible reason for it, but it happens!