Hi Bill,
If you are talking about custom headers/footers that you have created in the past, all you should have to do is clear out all the fields on the custom header/footer tab using the custom header/footer button. However, I suspect you are referring to headers like:
Prepared by Bill Wilkins 12/08/2001 Page 1
which should be in the headers dropdown list. This is NOT a custom header. Several headers and footers like this that contain you name and/or your company name will appear in the dropdown lists, and these are built when your file is opened. Excel looks at the name of the registered owner of Excel and the company name declared in the workbook properties to create these header and footer options for you, and I am not aware of any way to get rid of these. But at least they don't represent bloat or grow with time.
I hope this helps.
Damon