A more appropriate solution is to allow the checkboxes to be unchecked while on the page, and to use validation to ensure that submission is not allowed to occur when none are checked.
It's called validation, and can even take place before the submission occurs, so you can provide immediate feedback to someone when they uncheck the last box.
Consider, that with the current scheme when they try to untick the last box, and they're not allowed, that this breaks everything they have learned about checkboxes from the rest of the internet. The current system is a bad user experience for all of your users that come across it.
On-the-fly feedback is a much better solution that your users know and fully understand.