Doing

/tellraw (selector) []

will kick them with a big long message:

Internal Exception: io.netty.handler.codec.DecoderException: com.google.JsonSyntaxException:
com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1

This works with any other array of square brackets, ex.

[[]]

Now, this can be exploited by using a Repeating Command Block to do

/tellraw @a []

, effectively denying anyone from joining a server/realm/world.

This doesn't work with curly brackets, as it says "Invalid json: ".

The only good use for this is to give Realm owners a limited version of

/kick

to kick spamming players from the server.