Emacs & Autocomplete: disable RET to enter?

Emacs & Autocomplete: disable RET to enter?

I'd like to only auto-complete when I hit the TAB key, and not when
hitting RET. I've tried a number of config options, but I can't seem to
disable the action. Here's my current config, which contains all the
variations I've tried:
(define-key ac-completing-map "\t" 'ac-complete) ; use tab to complete,
working
(define-key ac-completing-map "\r" nil)
(define-key ac-completing-map "\C-m" nil)
(define-key ac-completing-map "RET" nil)
(define-key ac-completing-map "<return>" nil)
Interestingly, \C-m does work as expected, but hitting RET doesn't.
Can someone tell me what I'm missing?