I just tried to reply to a comment from a user with the handle:
Ciro Santilli 六四事件 法轮功 纳米比亚 威视
When I typed
@Ci the system autocompleted it for me, but when I finished the comment and pressed enter I got a big red box saying "Comments cannot contain that content". There was nothing unusual about the comment; I've written literally thousands upon thousands of them and never seen this. Removing the user name solved the issue.
This means it is impossible to respond via ping to that user.