From someone who's job was process improvement and re-engineering once upon a time this to me indicates deeper issues.
The major issue appears to be either training and/or usability. I think you need to get to the root cause and understand why they are going to these lengths and not using your workbook. If you understand why they are copying it perhaps you have a greater chance to resolve. They may for example, be altering it because the options on the Wb do not cover all the available scenarios, could be they have the same expenses each month so is easier for them to copy and paste certain data rather than enter it in every month, perhaps they are new and don't know the process etc.
You could go hard core and just reject the spreadsheet saying 'wrong form, use xyz' but perhaps this is an opportunity to see what can be improved and make yourself a hero by making things easier for them.
You cannot really stop people copying a file but via macros you could delete the workbook if it's not in a particular location.
regards,
Graham