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Not an easy one.

I presume that you mean automatically do this ? Manually you can of course ‘trash’ a topic and it will cease to appear on the forum

Also what would you like to happen to ‘closed’ topics, presume you would like to keep these, but in being hidden, presume you mean that they never show up in any forums/searches etc. but still show in the backend, or are you happy to actually delete them permanently?