Since a few hours something strange is going on…
Everytime someone gets kicked with Player:kick("…") the message shows for everyone.
But your are not actually banned, only the player the message was intended for gets banned.
From Steps to report a bug:
Is it level-specific? Is it game specific? Please post a link to the place that exhibits the issue.
Also, is this a message that can occur normally in your game, or is it something completely foreign? Are you sure this is actually a CoreGui and not a UI element of your game?