First contact local law enforcement and file a complaint. Then, contact an attorney who handles civil litigation. Many will offer a free initial consultation to outline your options.
There is no "specialty" in Bad Neighbor Law or Trash Prevention Law. I suppose one could say that a lawyer with experience in condominium or home owner association law would have more likelihood of having seen this kind a matter, but there is, quite simply, nothing specialized about it. What you need is an independent professional with a little good sense, a feel for the community health enforcement resources available in the District of Columbia, and, perhaps, a second-year law school knowledge of nuisance law. A licensed, local lawyer with some experience in real estate who feels comfortable in the area will probably be able to help. We deal with these sorts of issues fairly frequently, and it's often resolved with a well-written lawyer-letter or advice on where to complain.
Reading an answer on the Internet does not create an attorney-client relationship. You are represented by me when we have both signed a retainer agreement (on paper or electronically) and some money has changed hands. Usually, you will have been asked specific questions about your situation and all potential conflicts of interest will have been resolved. Until then, you have no more right to rely on this answer than if you read it in a novel.