[moderator note: split from here: More versatile Nyquist fade/amplify effect ]
Done.
That’s because the message was posted as a “Global” announcement, which makes it appear in all Forums, even newly created ones. Moderators/admins can solve that as follows. Edit the message in any of the places it occurs and change it to a normal “Announce”. That will remove the message from all Forums except the one you are in. You can then delete the message normally and it will be gone. Done.
Gale