Theyve Got The Wrong Person

I have been repeatedly called by a collection agency (Portfolio Recovery) for MONTHS and my credit report smeared due to an alleged 600+ purchase I made with this company. This account was allegedly opened by me over two years ago so you can just imagine what my credit report looks like. Not only did I never open an account with them I had to google them jusy to find out what kind of company it was. Of course each time I contact the agency I get a different rep, explanation, solution, etc, etc, ect. At this point I don't know what to do. Oh and get this...the rep I spoke to today told me it doesnt matter how many times they inquire about me, it wont affect my credit score. Of course she could only see one hit by them in her system...really? this is a nightmare!