If you want to keep the feature on but only delete certain entries, you can do this:
Go to the web site and select the input box that has history
type a letter or just double click to get the list to drop down
use the cursor keys to highlight the entry in the list you want to delete (do not use the mouse or you will close the list)
In IE, hit "Delete" key
In Firefox, hit "Shift + Delete"
that one item is now gone