User has to click button to mark task as complete....

Hi

I have a spreadsheet that users update which I then want them to perform a number of checks on. At the bottom will be a button for them to click stating "Click here to confirm checks completed". When they click it I want it to put a flag somewhere that I can then use to prevent them from saving if they haven't clicked the button. I know how to do the saving but, but how do I get the button to put a flag somewhere?

Thanks