It is an intentional feature that if you un-enrol a student, and the re-enrol them. The quiz data is preserved. (Image if the data was deleted in you un-enrolled the wrong student by mistake!)
Individual quiz attempts can be deleted if you go to the quiz results tab. You can select individual quiz attempts, and then use the delete control at the bottom of the page. I'm afraid I don't know much about the SCORM module.
How to reset a particular user's grades in one course?
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