That's not really true though is it?
If the announcement doesn't warrent any discussion then the OP can lock the thread (only staff can initiate threads in announcements, so they have this power).
If the thread does warrant discussion then one would expect that the OP will be reading the thread (or at least some member of staff would be) in which case they can remove unnecessary posts as they go.
I think that once the usual suspects realise they won't be able to gain copious amounts of kudo for posting nothings, they'll soon give up doing so. As it stands it's totally ridiculous, and any chances of any form of meaningful dialog taking place within those threads is lost to the sea of pointlessness.
As an example, about the only useful thing I have ever seen posted in response to the payback timings thread, is somebody pointing out an error (such as a wrong date). The same could be achieved by using the report button if the thread was locked. That thread does not need discussion, and posting thanks, or a tried and tested graphic for the umpteenth time is not discussion in any case.