How to remove a key from a minor-mode keymap in Emacs?

I have globally assigned C-c/ to ace-jump-mode but reftex-mode (a minor mode for citations used with AucTeX) overrides this key with some function I never use.

I tried local-unset-key but it only unbinds keys from the current major mode's map.

How do I remove C-c/ from reftex-mode-map without making changes to reftex.el?