This sounds like a job for The Extropian Pharmacist! (That's me.)
DepoProvera is the chemical castration agent of choice in treating sex
offenders.
Primary side effects are the result of blocking testoterone, allowing
endogenous estrogens (yes, males produce some) to act unopposed.
Increased body fat, loss of muscle mass, loss of body hair &
gynecomastia (female breast growth) are
most noticable; there are increased risks for various cancers, some of
which are promoted by estrogen.
A less drastic approach would involve trying selective serotonin reuptake inhibitors (SSRIs) such as Prozac,which have a high incidence of decreasing libido.
You might try consulting a reputable herbalist, as many herbs contain natural estrogens too, & might be suitable.