Cannot STOP Desktop and Documents sync

Running Sequoia on 27" iMac (2020) I accidentally switched on 'Synchronise Desktop and Documents to iCloud). I am getting iCloud storage full messages and when I attempt to reset the switch for this function, it bounces back to set.

My storage had used 38Gb of 50Gb Subscribed and my Documents + Desktop are way above the remaining 12.

I have tried saving my Desktop and Documents folders and recreating new, empty ones but the switch still won't switch off!

When I try to delete the Document and desktop folders on iCloud drive, they come back after 2 seconds.

Finally my configuration may have memorised something I don't want : I have

• an IMac,
• an Ibook,
• an older Mac running Catalina ,
• an iPad and
• an iPhone,

All on LAN and connected to one iCloud account