Make Shared Excel files only editable in Excel online, not downloadable.
Create an access level for Word and Excel files that allows the user to edit them online, but not download them to /upload a new version of them from their computer. Basically, online edit only.
Excel online only would be valuable. I often have 5+ people editing an Excel file in their native Excel application. If somebody leaves it open for a while, we end up with conflicting edits. That person has to save to a new file.... which leads to having to compare/contrast/reconcile those files. If we use online only, co-authoring works smoothly on a single document.