Not called - eunmin/Midje GitHub Wiki
If you want to check that a function is never called, do this:
(defn ostracized [n] (+ 300 n))
(defn myfun [n] (+ n 33))
(fact
(expect (myfun 3) => 36
(not-called ostracized)))
expect and not-called are part of semi-sweet mode, the framework upon which midje.sweet is built. Though not as pretty, you can use semi-sweet mode inside facts.