If my understanding of the Caching process is good, I'm pretty sure you have to flush all entries.
Let's suppose I'm the admin user and I desactivate the user named "test@test.com". 
If I only flush the cache entry "test@test.com", the suggestions for me (the admin user) remains the same. 
And I still have the deactivated user in my suggested friends. The example works also with the group-user-cache. 
Should I only delete the currentUser cache ? What's happening then for other users cached data ? ( assuming there is a few users simultaneously connected to the application ).