It sounds like you have done everything you can do.
I am a former federal and State prosecutor and have been handling criminal defense and personal injury cases for over 17 years. The above answer, and any follow up comments or emails, is for informational purposes only and not meant as legal advice.
Have you reported it to the state police?
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You cannot force the police to make an arrest. In theory, and I'm not advocating this, it's harder to decline to make an arrest if there is more evidence. If you have evidence of their ads on craigslist or backpage or some other proof that they are soliciting, that may help at least get the information sent to the vice squad for investigation. Also, does your local sheriff's office have a community affairs liaison? I find they are more receptive to community complaints; try speaking with them.
I am a former Brooklyn Criminal Court Deputy Bureau Chief with over 17 years experience specializing in handling criminal cases. All answers are for information purposes only. Answering this question or any future questions does not form any attorney-client relationship. Be mindful, that answers are limited by the limited facts presented by the questioner and are not meant to take the place of competent legal advice by an attorney fully informed of all the facts surrounding your case. Also, be aware that nothing posted in a public forum such as this can be deemed confidential or privileged communication.