Lock current file format extension on save as dialog box

I have a few files sent out to field users. These users, for some reason, really love to change my .xlsm excel files to .xlsx when they save. Is there any VBA code that will help with this? The closest I could come up with is using the BeforeSave workbook event, but this still allows the user to make any changes they want to extension once the Save As dialog box appears.

Example of what I want:

User clicks save as, types in a new file name and selects .xls
Macro assigns the previous file extension to the file then saves it.