Some good comments Alanabit.
Essentially, prostitution to feed a drug habit is a side effect of the drug culture, to combat that we need to find ways of effectivly countering that culture.
Broadening the discussion to include this related problem, my ideas on countering illegal drugs include stigmatising the social use as well as providing better and faster support for addicts.
We need to put in place a system of clinics where addicts may register, then receive supplies and be supervised in a safe, hygenic environment.
Other measures could then be put in place to attack the drug user as much as the pusher.
Regarding the effect of this on prostitution, there will be at least a small chance of getting the girls who are on drugs out of the reach of the pimps.
There will, of course, still be the problem of "white slavery" with girls from overseas being tricked and forced into prostitution, this will of course require better immigration controls, something we can whistle for with this excuse of a government.