Unwanted Time Machine backup

A few days ago my wife reported that TM kept giving her reminders that she had not backed up for how ever many days and could I stop that. I had a look at her TM settings. She doesn’t use it at all at the moment, just a CCC clone on EHD. There doesn’t seem to be an on/off button on TM in High Sierra. The box for automatic backups is unticked. I then got a dialogue box asking if I would like to use the CCC Clone EHD for TM backups, naturally I said no. The next day we found that TM had performed a new backup to her CCC EHD.
As I don’t have a spare EHD we will need to wait until we are home to completely erase the EHD and perform a new CCC backup because I can think of no other way to get rid of the TM backup which has nearly filled the EHD.
I don’t understand where the on/off button has gone or why it would backup to a drive I specifically asked it not to. Bit of a nuisance when you are away from home.

I opened the TM application and there is a box labelled "Back Up Automatically" there. I have mine un-checked and TM does not run automatically. Not as good as an on/off setting, but that is apparently how one does it now.

EDIT: I clicked on the "?" and that is confirmed by the resulting help box.

Ah well, I guess it will go down as an unexplained glitch. Currently up in Sydney for a wedding, tomorrow back to Bali. So I will check it out then. Annie has a TM backup there so can update that, erase the current backup and create a new CCC clone.