What is the process for cleansing a very negative action?

Several years ago I did something that was totally legal, but very much against my upbringing, my world-view, and my ethics. It was a spur of the moment thing and I have regretted it massively ever since. However, it has haunted me mentally (and to an extent physically, due to tension) ever since. I might add, this event didn't hurt anyone else. What can I do from a Buddhist perspective, to completely cancel out this action and its resultant negative thought-stream once and for all?