Last year's AP Packet said that scores can't be canceled after June 15th. This year, the website says "Grades may be canceled at any time. However, for grades not to appear on the current year's grade report, AP services must receive a signed letter requesting cancellation by June 15. While there is no fee for this service, your exam fee is not refunded. The grade report that you and your school receive will indicate that the grade has been canceled."
I'm guessing that non-seniors who cancel their scores after they receive them will still have the scores on this year's score reports, but the score report they get next year will not have the canceled score.