You can’t delete an Announcements Forum without first changing a setting in Course Admin > Edit settings, then scroll down to Appearance and change the “Number of announcements” to 0. If that number is greater than 0, when you delete the Announcements Forum, it will automatically be recreated the next time you load the course page.
(Even better, would be to do that and then go to Site Admin > Courses > Course default settings and change it there too - it won’t help with Announcements Forums in existing courses but it will stop them being created in any new courses.)