This goes against logic but maybe it's time to approach this in a different light to bring them out of the closest. One time partial amnesty. If caught using a banned substance for the first time you would have your suspension reduced by 50% if you cooperate with authorities and turn on your supplier. The supplier can then negotiate with authorities by turning on others he supplied. Next offence nail them all to the wall.
Some might say that no one would turn on a "buddy". I say bull, these guys are all about money and cheated to begin with so integrity is not an attribute high on their list of values. Soon there will be fear running throught the system at the user level and thats where you want fear to reside.