Because of how I approached a problem where a bug can relocate a site to Africa when its boundary is subsequently modified, and the same can happen when a replacement area is added to an existing activity, I now have a couple of legacy sites after I have recreated them as a new activity with a newly input site boundary. No records are associated with any of the activities in the legacy sites (ie they currently include any records by anyone without joining the activity (in the past one of them may have required people to join and post into the activity).
However I can't see how to deleting an activity? For example to help people not join an activity that is not maintained.
(So instead I've marked them as legacy [title] and (I hope) made them invisible to anyone but administrators)
Untidy but Yes!
… that will work.
For those that have been used by others but superseded (because of a very crudely drawn boundary) it is right that it can't be deleted, but there is some form of alert that there is an updated version. So I have changed the title to "Legacy [Name of Site]" and there is now a new area with the same name and which will pull in all the records.
I had an activity that I'd set up and never used, no other people using it. So I removed myself from it. It has vanished from the list of all activities. I suspect it is still in the database, but as long as it's invisible it is effectively deleted.