It's not possible without actually modifying code to do this safely, though I assume sending a corrupted MOTD and cancelling the ping might cause the client to display the server as offline.
I just tested and it seems passing in a null MOTD will cause the server to appear offline. Players can still connect to it though~ Oh, and it will display a short 1-line error message in the console each time the server is pinged and returns the null MOTD xD