A message like this would probably fit into the console:
IP address 123.123.123.xxx has been blocked. Disconnected players: Alice. Other players affected: Bob, Cari
If it means too much work to determine the players who get disconnected or are currently (still) connected but will be affected by this block if they get disconnected, then a message like this (similar to what we have now) would be sufficient:
The server manager has blocked IP address 123.123.123.xxx and kicked a player using it.
This wording isn't great but a compromise between properly and understandably explaining what has just happened. Better suggestions are welcome. The wording we have so far (3.6.0.3) has made at least two people believe they had been blocked when someone else had actually been blocked.