MRS is total scum, of the worst variety. Been there, done that.
Did you send a demand for validation in writing, CMRRR, and within the 30-day window? That's usually enough to back them off. Hit'em one more time with a strongly worded DV letter. And be prepared to follow up with an ITS if they don't provide satisfaction. Also, be advised that MRS is known to front for LVNV Funding.
When we were dealing with MRS, they were calling on some bogus CC account that my wife never had. Some puffed-up city boy from New Jersey thought he was going to intimidate me. Didn't quite work out that way. I just gave him back the same, in spades. Had him ready to bust a blood vessel a couple of times...
We didn't have an OC or account on the bill they were calling us for, and they refused to provide any information. So I DV'ed them rather harshly, referencing the account as "any and all business you may have with me/us." That letter got the account pulled from MRS, and sent to Resurgent Capital. Those folks gave me the OC and account number, and I social-engineered the fact that LVNV was actually the account owner out of them. At that point, I sent the same letter that had worked on MRS to
both Resurgent and LVNV. Haven't heard peep from them since. Two years and counting.