High doses of dutasteride bind for a long time to 5-AR (type I AND II) and essentially inactivate it... Serum DHT is reduced by 96% on high dose Dutasteride. Scalp levels are reduced by 82% (if you believe the studies) with 2.5 mg dutasteride a day. Im not sure if "killing" the enzyme and further reducing DHT would have much more effect on DHT than that currently available with high dose dutasteride. At the levels of supression available with Dutasteride 2.5mg/day testosterone may in fact be the major active androgen in the follicle.
PS- oral medications that nonspecifically block the androgen RECEPTOR are available and are used for cancer patients... the side effects are that of castration as you are blocking testosterone and DHT nonspecifically throughout the body (follicle androgen receptors are the same as those throughout the body, unfortunately). Topical androgen blockade is possible (5FU, spironolactone, fluradil) with possible benefit and without side effects...