You can cancel your GMAT exam scores only at the test center, at a very specific time: right after you finish the exam, but before you see your scores. You cannot cancel your exam scores after they are presented to you, or after they are sent to you or to the schools you chose to send them.

If you choose to cancel your exam scores:
You cannot reinstate them.
You will not be eligible for any monetary refund.
The score cancellation will be stored as part of your record, which will include all your future scores.
You must wait 31 days before retaking the exam.

Any other cancellation of your GMAT scores is at the discretion of GMAC and/or Pearson VUE.

If you choose not to cancel your scores:
At the test center, you can view and print the unofficial scores (scores for the quantitative and verbal sections, and the total score).
Your Official Score Report, which also includes your score for the analytical writing section, will be available to you and to the schools you chose to send the scores within 20 days of the exam date.