If you're using Internet Explorer, the function that does that is called 'Auto Complete'. You can clear the Auto Complete history by choosing "Tools', then 'Internet Options' from the drop-down menu. That opens a box that has tabs across the top. Choose the 'content' tab, then choose 'Auto Complete'. It's got instructions listed for clearing the history (something about going to the 'General' tab, choosing 'delete' (under the 'Browsing History' heading) then choosing 'delete forms' or 'delete passwords').
I don't believe that using auto-complete counts as a mechanical entry, but I'd be interested to hear from others regarding that, because I mostly use auto-complete when entering sweeps at my work computer.