Honest ! I've really spent ages, before writing to the message board, going round and round the relevant- and irrelevant- pages trying to find a way to remove this tree. I've also followed the instructions given in this thread to the letter. I wrote to the tree owner with little hope, got the reply a few minutes ago and looked immediately at both the app and the Home page and the tree has gone, leaving no trace.
I've never shared a tree on Ancestry before as I usually do that on Genereunited so I have no experience of removing one. I have used Ancestry for many years however so am not a complete newcomer to the layout.
As you say...a mystery and I'm grateful to the tree owner for his prompt action.