Prevent excel from updating external file links

I have a bunch of excel files and they all link to 3 other excel files on sharepoint. The downside is we have maybe 4-5 people who access these files. For some reason and I dont know who/what is causing it, but the links to the external files sometimes get updated by excel. When excel changes these links, it changes them from the http sharepoint to C:\ instead which causes issues since that is not where the files exist.

I am guessing maybe someone is occasionally saving the files on their hard drive which is causing all the external links to swap to C:. Is there any way to stop these links from automatically changing like this? These files are linked in formulas and/or buttons linked to macros on another file.