My goal is to create templates in Word 2016 that enforce following a set of fonts and sizes, but I can't figure out how to delete the styles I don't need, or how to restrict the user to use only those styles.
Similar questions to this have been asked before, such as:
Force style usage in Microsoft Word
but these pertain to older versions of Word and I can't find any similar settings in office 365. I'm using word for Mac, if that makes a difference.
You can't author the restrictions in Word for Mac - it currently doesn't have this functionality. It will, however, honor any restrictions that are defined in a word file. So if you create the documents and enforce the style rules from a Windows copy - you can use that file on Mac and get the result.
Microsoft uses a site called UserVoice to track requests for certain features. If there is enough push for it, they will get it on the roadmap to implement. Right now, this particular item I found was requested - but it has not received much attention. You can add your vote to it here though: https://word.uservoice.com/forums/304942-word-for-mac/suggestions/13414068-https-support-office-com-en-us-article-restrict
