You want these to stop, no matter what you do it is unlikely they will.

Instead you can change how you see it as just a message, he doesn't keep doing it and putting on pressure, he doesn't get anyone else to pressure you either. So you really could just keep one email channel open, put a message saying "thank you for your email". And just keep ignoring it.

Keep in mind that you are in control of responding back, so he gets nothing in return for his messages. I wouldn't even block them, just regard them as an annoyance that you have to be reminded he exists a couple of times a year.

You could create an email account that would just route the emails into a black hole, pretty much the same as now. Block everything else but leave one channel he can send to and think that you are receiving them.