@FloofyNewfie Adding your explanation here. Maybe it should also be added somewhere more visible.
It leads me to a question though, what if you do both the ignore buttons but then want to take it all back?
There are two separate Ignore buttons:
1. On the user profile page
2. When leaving a PM conversation.
Clicking Leave and Ignore Future PMs (leaving a conversation) alone will still allow the other user to send you a new separate PM.
Clicking Leave and Ignore Future PMs and then clicking Ignore on their profile itself stops all PMs from showing up. Even when unignored the second PM didn't show up.
So as long as you leave the conversation, ignore future PMs, and then ignore the user from their profile you will not get future PMs, and the person sending the PM will have no indication that they've been blocked.
If you just ignore them, the PM will still show up, but when you click on it you'll get the generic "Show hidden content..." button. And if you select leave but select allow future PMs. If the user replies to the same PM thread it'll show up as a new message, but will be hidden by default.
So the only way to block all future PMs is to leave, don't allow future PMs, and then ignore the person's profile.
(This is how the forum works, this is not programmed by mods, so please do not complain that this is convoluted.)