How can I permanently delete bookmarks from Safari?

There are several bookmarks in Safari (Version 5.1.10) on my iMac that are obsolete and others in which I am no longer interested. When I delete them, either using keyboard commands or by going to "Edit: Delete," they return. I have tried putting them in folders, out of the way of the main bookmarks page. I have pulled them off of the favorites bar. They always come back. I really want them gone.

My iMac has a 2.7 GHz Intel Core i5 processor with 4GB 1333MHz DDR3 memory. It is running OS X 10.6.8.