John,
I don't have this problem, so I can't say for sure how to stop this.
My guess is that you've got a cookie lying around on your computer somewhere.
If you use MS Internet Explorer, look for a folder labeled "Cookies". Delete anything that has "ebates" in the name. Also look for a folder labeled "Temporary Internet Files" and see if it contains a "Content.IE5" folder or some such. Again, look for cookies with "ebates" in the name. Delete them.
Also, email Ebates and let them know how annoying this is and ask how you might stop it.