Option to disable document library check out / check in functionality

There is no way that I am aware of that you can do this.  Even if you added some sort of script to the page, there are still other methods a user can checkout and check in documents.

If a user opens a document in an Office application, they can check-out the document from there and no amount of SharePoint customization is going to stop that as you are no longer in SharePoint.  So even hiding the buttons isn't going to completely remove the ability to disable the feature.

I am not sure if this can be customized in the new format as the library reverts to the "classic" mode if it detects custom development modifying some of the features (https://blogs.office.com/2016/06/07/modern-document-libraries-in-sharepoint/).

Edit: I know you can do this to a point in SP2013 classic library (http://bliblablog.net/hide-check-out-in-sharepoint-2013), but in the case of the new libraries, I am not sure if that option is available (haven't played too much with the new library yet).  However, from what I have seen there is no straight forward way to disable the options in the library.  Again, even if you are able to, the controls you build will not affect the Office Integration options.