I'm surprised I've never really needed to figure this out until now:
On a Marketo landing page, I need to add a "Not you?" button so that the user can clear out the form and re-enter new information, on the same page.
Yes, this already exists -- but only if you use Known Lead HTML ("If Known Lead, Show Custom HTML" option in Form Settings).
So a code-free way to make it available is to add a second form on the same page. That second form has KL HTML enabled, and everything else in it is hidden with CSS.
I can also show you how to redo the same logic in custom JS if necessary.
there are several options, I believe some solutions are on the forum or my website.
Yeah - Josh's suggestion of just making a 2nd page that has the pre-fill disabled was my fallback. I might do that to get this page out the door. but I think, going forward, using "If Known Lead, show x" option in form settings might be the most appealing. I'll see what I can figure out, and get back to ya.
The hitch I see in just disabling PreFill is you're letting somebody say "Forget whoever you think I am" but then not actually forgetting them on that click (i.e. on PreFill pages and other pageviews, they'll still be the last person until they reassociate themselves).
For that reason, the native Not You session regeneration link actually forgets them on the spot, even if they don't reconvert.
I was able to convince my team to go with the option to show alternate html if known lead. That way, the user's not given that option to be forgotten. So that's my solution for now!