Well adly3000... i like the idea! Anyway, a feature like this (closing the topics if that's the case) would cause a user to open a new topic (duplicated) and perhaps with a stupid question that he/she was not able to conclude in a closed one.
And suggesting/asking a user to notify the other posters will be a waste of time. A great example: we all know the main rules of a BBS, but... cross posts and the no use of code tags... well... they still
However, this can be a great PHPBB hack, pluggin... whatever! Believe me!
Is there some adventurer here !?