I’ll start with a simple trick. Discourse allows moderators to set a timer to close or delete a topic automatically.
Here is a real example: Forum Weekly Report - #33 by NPhan_WMF
The Forum Weekly Report has been active during the community review, but now it’s over. We have moved to a monthly cadence. Nhu says that we will be closing this topic soon. At that point, moderators can set a timer to close the topic automatically (in this case 7 days) and forget about it.
It is possible to set timers relative to the last comment. For instance, you can set a timer to close a topic one month after the last reply. If it’s been 27 days of silence but then a new reply arrives, the timer is reset and another month needs to pass to close the last topic.
You can even set a timer at a category level. For instance, imagine a “Hot Topic Only” category. If a topic is not hot, it doesn’t have a place there. Very well. You can set a timer for that category so that topics without any replies after one week are closed automatically.
You can also set a timer to auto-delete comments on a topic. Why someone would do that? Imagine a topic “Forum help, just ask!”. Some people ask questions that haven’t been asked before, and moderators can split those questions into new topics that will be useful for future users. However, some people ask questions that have been answered before, maybe many times. The replies are polite links to existing topics where these questions are answered. Instead of accumulating these comments that after a while become noise, you can set a timer to auto-delete replies after one month. If a reply was not informative enough to be moved elsewhere, it will be removed, and the topic will stay pruned and fresh.
(This was supposed to be a tip of a couple of lines, but I got carried away! 
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Share your questions, answers, tips and tricks!