I recently had a young reader write in and ask if Saw Palmetto could prevent facial hair growth and development. What he had noticed is that he had had very little facial hair growth for the last year or so since taking Saw Palmetto and was wondering if there was a correlation.
Saw Palmetto is known for its DHT-lowering capabilities and has been used to treat enlarged prostate and hair loss, two conditions related to DHT levels. In fact, some experts have observed that Saw Palmetto not only lowers the rate of DHT formation but also its binding abilities as well.  However, in spite of its superpowers, clinical use of Saw Palmetto has been limited and results controversial.
In the case of hair loss prevention, there is really only one legitimate study using Saw Palmetto and it was combined with another ingredient (beta sitosterol).  So the bottom line is that no one is really going to have a definitive answer to this reader's question. However, there are some things we can say, such as the fact that DHT is clearly strongly involved in the growth of facial hair. Children with a DHT deficiency will have dramatically reduced body and facial hair (and testes/prostate development). 
There is also a study on celiacs that shows that testosterone levels in young males is responsible for hair density, but DHT is responsible for hair growth.  Therefore, there is some good clinical evidence that Saw Palmetto, or any DHT-limiting herb or drug, could reduce facial hair growth.
Indeed, this is a widely reported side effect of Saw Palmetto usage. In fact, one expert notes an unusual property of Saw Palmetto: it promotes hair growth on the scalp but reduces it on the face.  So in all likelihood his loss of facial hair growth is probably due to the Saw Palmetto but I certainly cannot "prove it" with a study.
CAUTION: Some of you may wonder why a young guy would take Saw Palmetto in the first place, because they rarely have hair loss or enlarged prostate? The answer is often steroid use. Some of the steroids will raise DHT levels and young guys will take Saw Palmetto for preventative measures. (They will also often take estrogen blockers as well.)
1) INTERNATIONAL UROLOGY AND NEPHROLOGY, 33(2):217-225, "Randomized trial of a combination of natural products (cernitin, saw palmetto, B-sitosterol, vitamin E) on symptoms of benign prostatic hyperplasia (BPH)"
2) Journal of Clinical Endocrinology & Metabolism, 76:524-528, "The androgen control of sebum production. Studies of subjects with dihydrotestosterone deficiency and complete androgen insensitivity"
3) British Journal of Dermatology, Nov 1982, 107(5):559-564, "Relationship between plasma testosterone and dihydrotestosterone concentrations and male facial hair growth"
5) The Journal of Alternative and Complementary Medicine, April 2002, 8(2):143-152, "A Randomized, Double-Blind, Placebo-Controlled Trial to Determine the Effectiveness of Botanically Derived Inhibitors of 5-?-Reductase in the Treatment of Androgenetic Alopecia"