First, consider getting people to actually use it as a template. This does not involve opening it. Rather a new document is created based on the template.
Templates in Microsoft Word

It would certainly be possible to put a macro in the template that forced this, but then you have the problem that users do not have to allow macros to run. I can imagine a number of methods, but the one I started with is the most practical from my point of view.

One possibility would be to mark the file as Read-Only in the Windows properties of the file. This is done from Windows, not Word.