Interesting question.
Collect all the ugly men pictures and have her see those every hour or two.
She will never think about sex again!!:)
Being overweight may reduce libido. So feed her fatty food!
On a serious note:
Please read about medical castration.
Its the testosterone in small amount which makes women (men) libido stronger. So blocking it reduces libido. Actually both in men and women.
These meds are commonly used for cancer patients to kill cells affected by testosterone.
One group is LHRH agonist like goserelin
or anti-testosterone like cyproterne.
She may have hot flashes like menopausal women.
(On the same token, testosterone patches can be used to INCREASE libido.
With possible side effect of losing testosterone permanently if used by normal people)
It has also been shown that women who take contraceptives for a while and even if they stop these pills, it leads to reduced libido. :(