A while back, I provided a link to a script to show an example of how to clear the run history. I still get a lot of questions about the Run History, and clearing it. I have put this document together to hopefully help answer the questions about clearing the run history.
How to clear the run history: http://social.technet.microsoft.com/wiki/contents/articles/7034.how-to-clear-the-run-history.aspx
The document will go through and talk about the different ways that you can clear the run history, and also has some common questions that are asked to me about the run history. I hope that this helps.
Feedback is always welcome!