I could give you some complicated instructions on how to remove it from the MySQL database, but that could potentially run into quite a bit more of a mess than what I stated above.

The wording of this link has changed in more recent versions, it now changes the wording if it's already an announcement or not. Both announcements and sticky posts are handled by the stickpost/unstickpost scripts as:
ubbthreads.php/ubb/stickpost/Number/threadid/announce/1
ubbthreads.php/ubb/unstickpost/Number/threadid/announce/1

So you could make the link yourself if you really wanted.

Also, you should look at upgrading as there are some security issues with the version you're running.

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