How can I remove an negative item from my credit report?

About 5 years ago, I made the mistake of co-signing on an auto loan for a girlfriend. At the time, I thought I was going to be a back-up co-signer and not a full signatory on the loan. Anyway, I soon learned that I was wrong and the loan was in both names. As you probably expect, the relationship ended and she decided to exact her revenge by not paying her loan on-time. Since the relationship got ugly, I ended up working thru a policeman who would call her to make sure she ended up paying the loans but if I called her directly or thru friends, she would say I was harring her so I had to work thru a friend at the police dept. While the loan has now been paid in full, I notice on my credit report that I have three negative items, all from her lack of timely payment on the auto loan. Is there anyone that can offer advice on how I can get these negative reports removed from my credit record????