Happy to try to help, but can you define what you mean by ‘clean-up’ in this case?
One of the strengths of a forum is that relevant content can be found over time and does not have to be recent - the problem, for example of social networks like twitter.
The content will be available for users to search and discover, so it is more helpful to do the work to link & merge related topics as they arise to create better reference content rather than ‘clean up’ afterwards, but it will depend on your community and what content is being created.
One thing you could consider is in Category Settings, you can set Auto-close topic hours to close automatically after a certain time, so new arrivals can read the material and reference it but not revive the topic. Have you tried this?
If you want to select which topics are “unnecessary / outdated” you can select them and close them all - there is more information on that here: