You didn't indicate what application your user is creating content from and/or what is being used to create PDF. However, there are two factors here.
The first is PDF creation. Your PDF creation capability may have an option to explicitly enable links within the PDF file. If you have such an option, obviously you can turn it off.
The second factor is the PDF viewer. In Adobe Reader and Adobe Acrobat, both on Windows and MacOS, there is a user-settable option to Create links from URLs. If this option is enabled, heuristics are used to recognize URLs and make them clickable when displaying PDF files. Disabling that option in these programs, turns off that automatic URL recognition and only supports clicking on URLs that were specified during content and subsequent PDF creation.
The problem here is that other PDF viewers may automatically recognize URLs or Mailto: links and not provide any capability of disabling them.
Bottom line is that there is absolutely no way to totally disable clickable links because of such automatic link recognition of various PDF viewers.
Sorry!
- Dov