Bruce,
Thank you again for your interest in Vacation Request!
For an approved Leave Request there is a Revoke Leave function/button which pretty much does what you have asked for. Goto the Leave Request Forms View, find the Leave Request for the already approved Leave in question, and click the Revoke button.
Also, in the next release (ready but not throughly tested) I have added a Cancel Leave function/button for cases where the Leave has been requested but not yet approved and the Requestor changes their mind.
HTH,
Bob