Are you anticipating paying an attorney to do this? You're going to have to pay an hourly rate because I don't think you're going to get counsel on a contingency fee - because you don't really have damages. It may be that you can ultimately have a claim for civil penalties (as described below) but that's not entirely clear and I'm not sure any attorney is going to take that case on contingency.
But you can move the ball here by yourself. If you're dealing with a debt collection agency, you have rights under the Fair Debt Collection Practices Act. You can find resources online to familiarize yourself with it. But basically you can send another certified letter demanding that they properly validate the debt under the FDCPA and that they remove any negative reporting of this debt immediately. You can advise that your identity was stolen and that you did not rent the apartment. You can demand that until the debt is properly validated pursuant to the FDCPA, any further attempts to collect the debt (including reporting of the debt) will be subject to civil penalties per occurrence and that you will be retaining counsel if this matter proceeds any further.
Then you can keep strict records of any phone calls. Document names, dates, times and content of call. The penalty for improper debt collection is something like $1,500 per occurrence. If they don't ever call you back and remove the report, you've defeated them. Make sure they remove the line from your reports. If they do not, write them back and demand that they do or you will be suing for violation of the FDCPA.
If they don't send back validation but continue to try to collect the debt, you may be able to get civil penalties for each time they harrassed you. But I'm pretty sure that until you demand validation, access to civil penalties (for the stuff they have already done) is pretty limited. At that point, however, you may have a case worth taking on contingency.
If they do send you back a properly validated debt (it may be possible for them to do that - the problem here is that it just isn't you) you may need to work with them on how to prove it wasn't you - or to sue them to get a court to make that determination. You could probably do that in small claims or pay an attorney.
Of course, that's just my academic opinion and should not be treated as legal advice. You should always try to get a free consultation - most lawyers will give free consultations and tell you whether there's a case they're interested in working on.