That’s interesting that you are able to activate a staged user via their user admin page. Staged users are not expected to log in so it doesn’t make sense to be able to activate them. Staged users can transform 
into users by them coming to create accounts, and then they gain access to all the messages they only participated in previously by email.
So.. one answer for you is to just not do that. 
When you want to invite someone to your site, just invite them in the usual way - not by creating a staged user via email.
But maybe we can prevent this behavior in case others encounter it. I will do a quick test on my site to see if I am able to replicate.