Over moderation is what kills everything. Doesn't matter what it is it removes community interaction. You have a choice to close off topics because they're off topic, sure that's fine, but when it's on topic but bordering on becoming derailed don't just lock it straight away, give it a chance to come back. Not everyone is a master of posting nor are they a perfect poster. Everyone has garbage posts at one time or another, some more than others. Simply put, let those grey area posts of near derailment have a chance to get back on topic rather than locking them because they might go off topic.