I create a new sheet for every month and record my income and bill payments and it works really well.

However, I would like to be able to mark sheets as "complete" and then seal them from editing with a "lock" so I don't ACCIDENTALLY edit them in the future.

The lock I am envision is as simple as lock icon on a sheet that you click and it asks if you want to lock it or unlock it.

No password needed, just an ability to:

1.) mark as complete and

2.) stop from accidentally being edited.