One used to be able to block a user with a Private Profile by going to preferences/users and clicking “add” under ignore, then entering their name-server, which can be found in their profile URL. This would populate the “add” field with the appropriate character and allow you to block them.
Now, if the use has a private profile, the “add” field will NOT populate with any users if they have their profile set to private, even with name and server. You can search for any non-private character this way, so currently, there is no way to block a user with a hidden profile.
Edit: Playing around with this some more, the behavior is very inconsistent. Some users will be found if you change the first letter from a capital to lower-case. Others will not be found regardless. I’m not sure if this is an issue with “special characters” like a ‘š’ or something similar but it doesn’t appear to be.