Unfortunately, I don't think there's any easy way. Implicitly detecting URIs happens at the reST-parsing layer:
Sphinx is a set of predefined domains & related tooling on top of docutils' reST implementation, so this is lower-level than the things it provides config options for.
There might be some way of getting the HTML writer to not emit <a> tags out the output side of things, but my guess is that even if it's possible, it's likely to be pretty involved.