This is done with cookie data. So you need to clear cache to remove the data.
You can disable form fill at the form settings page
Assuming you're talking about a Marketo-hosted LP, this feature is called Form PreFill (and must not be confused with Progressive Profiling). It can be enabled/disabled at 3 different levels: for your whole instance in the Admin section, for an individual form field in Form Editor, and for an LP in LP editor.
The reason you may see unexpected data is if someone has forwarded or tested an email on a computer other than the original recipient's device(s).
Ryan, I would not clear the browser's entire cache or cookie store just to fix this. That can wreak havoc on the lead's stored credentials on other sites and leave a very unhappy camper. The only cookie that needs to be deleted is the cookie `_mkt_trk` for the current site.
Also, while cookies are used to assist with PreFill across page views, PreFill doesn't fully depend on cookies. For example, a clicked email link will enable PreFill before any cookies are set.