How often can you dispute an item on your credit report?

If you don't use the same reason over and over again, you can generally dispute a tradeline 3-4 times over a period of say, a year, before the CRA's will deem your disputes frivilous. They may then add a 7 year extended fraud alert to your reports and move you to their special handling department. For TU and EX, I think they keep a PITA file and move you to special handling quicker than with EQ (CSC)

Here is a three step method of dispute copied from the law office website. Once you have followed the three steps and failed in the courts the reporting agency does not have to follow up on another challenge. If you just keep challenging and failing they will, as nascardevil says, not take you seriously.

Letter 1 - initial communication and request for correction of inaccurate or unverifiable information

Letter 2 - if you sent Letter 1, and more than 30 days have passed since the date your letter was received (per the certified letter receipt), Letter 2 demands immediate action

Letter 3 - if you receive a form letter back from the credit bureau stating that they have conducted their investigation and that the challenged information is accurate, send this letter to demand that they reconsider their position

A persistent and consistent letter writing campaign should be effective in correcting most errors. If errors remain despite your letter writing efforts, you do have the right to file a lawsuit in federal court to ask a judge to order the credit bureau to change the information in your file.