To prevent very old posts/ideas, say older than 1 or 2 years, being bumped to the top of the recent list, prevent commenting on these, for example.
The content may still be relevant though, so you can reference the post/idea in a new one thus linking back to the old post/idea.
Also, when viewing the initial post/idea, perhaps show any referencing posts/ideas so that users can navigate to the latest conversation around it.