Is there a setting I could apply to a user that would make user unmentionable?
The reason is - we are primarily a physicla community with strict membership system (people need to pay fees). When people leave, we suspend them and od not delete them so their message history is preserved. I would like to remove them from the list of mentionable people, so people cannot mention them thinking they are still members. Is it possible?
Maybe add their name to Watched Words so it can’t be posted? There has to be a better way, though.
Just for a bit of extra detail on how it currently works. 
For non-staff, a suspended user doesn’t show up in the @mention autocomplete that pops up when you start typing straight after the @, which may be helpful to dissuade regular people from trying to ping them. 
And if you don’t hide suspension reasons, their user card should look a little like this when clicked on:
That’s if I suspend them ‘forever’ and add a custom ‘no longer a member’ reason.
If you only ever suspend users due to expired memberships you could even amend the respective site texts and have it look more like this:
For that I updated js.user.suspended_permanently and js.user.suspended_reason, and added the date using the custom reason option when suspending the account.
Though you could go one step further and also deactivate their account as well. That would then hide their user card for normal users and replace the info with ‘this user is no longer active’:
So if people do manually type the ex-member’s @mention even though it’s not in the autocomplete menu there are still a couple of ways you can let people know they’re no longer an active member.
Aha, indeed it was a moderator who was complaining about this, I did not realize it was different for ordinar6 users. Thanks for all the other tips!
You can offer to revoke their moderation. 
