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

You can change an existing key map using define-key. By passing nil as the function to call, the key will become unbound. I guess that you should be able to do something like:

(define-key reftex-mode-map "\C-c/" nil)

Of course, you should do this in some kind of hook, for example:

(defun my-reftex-hook ()
  (define-key reftex-mode-map "\C-c/" nil))
(add-hook 'reftex-mode-hook 'my-reftex-hook)