You can have ACT test scores deleted by just writing to them (as long as the scores aren’t already enroute to a college). Just complete the form that they send you and mail back.
http://www.act.org/content/act/en/products-and-services/the-act/help.html
“. . . you must submit a written request. . . We will then permanently remove your record for that test date from our files. All scores from that test date will be deleted.”