Automatic Windows 11 upgrade issues

My customers have begun calling in saying their Win10 Pro PCs have upgraded to Win11 without any interaction on their part.  None of these are on a domain, but rather are just standalone PCs in a small office.  Questions:

1) Is there a way to prevent this from happening any time in the near future?  If so, exactly what do I need to do and where in the system?

2) Is it safe to revert these back to Win10?  I see on the Go Back dialog where it says "You'll have to reinstall some apps and programs".  Note that they don't say "only if they were installed after the upgrade".  This makes it sounds like it is a given--that you will have to reinstall no matter what.  Is that the case?

TIA