I don't want iCloud - how can I reject new T&Cs and stop being asked to accept them?

Turn iCloud off.

System Settings-->Apple Account--> Turn off everything. Photos, Drive, Passwords, Notes, Messages, Mail. Also look under Manage: Most of them will ask you to delete them from iCloud.

After you shut everything off, then go to  > System Settings > [name on top] > iCloud > Manage, and choose each app there to remove any data that still remains in iCloud.