This is already happening out of the box. People only see activity in categories they have access to in their email summary. It is delivered weekly by default.
That’s not the behaviour we are experiencing. People who are members of Group A are getting activity in their digest emails from categories only accessible to members of Group B.
The reason we know this is because a member of Group A wrote to ask why he was seeing posts from categories accessible only to Group B.
You will want to take a closer look at the category permissions. If it is set up right it will behave as I have described… unless you have discovered a bug.
A situation I have run into before is that the category permissions are sometimes confusing to change. You think you are limiting access to one group but the previous security settings remain, eg including everyone.
Or maybe you are running into a situation with an admin who has access to everything. You can’t limit access to admins at all.