Today I released version 1.0.0 of Guillotheme, a plugin that chops the front-end (i.e. the head) right off your theme by restricting access to the front-end.
Specifically, it does this by using conditional (and customisable) redirects, to prevent accidental access to WordPress-generated front-end permalinks. These are unnecessary in the case of a headless or decoupled set-up, and could potentially confuse non-technical users or other content editors when WordPress is being used headlessly (i.e. purely for its CMS capabilities).
It can be downloaded for free from the official WordPress repository.