You can delete chat by following these steps:

1. Tap the Chat button in the upper right-hand corner of the board.
2. Tap on the Shield icon in the top-right corner of the screen.
3. The options window will then appear. Simply choose the “Delete All Messages” option.
In addition, when you log out and log in again to your account the deleted messages will be visible again to your part because that option is not permanently deleting the messages. After some time, the system will permanently delete the messages that you have on your end.