Wistia-Marketo form field mapping

Did Wistia create that person in question? If so, there must be a Wistia form in play. And that form should have that Lead Source mapping.

When I did Wistia integration, we didn't use Wistia forms, so the only thing the integration gave us were "Visit Web Page" events. And these events don't write into Lead Source per se. But you have a Smart Campaign somewhere in your setup that does that. Did you check the dependencies on the Field "Lead Source" to see if there is such a process?

Wistia isn't a Marketo launchpoint partner, and IMO most of the integration nitty gritties, including what field/data it writes to, field mapping etc. is managed more-so by the Wistia itself. AFAIK - with the integration set and enabled, when a new lead is created through Turnstile forms, the default behavior of the integration is to create a new person in Marketo with Person Source (LeadSource) field set to “Wistia Video: <name of the video>.” Do you see Wistia updating the LeadSource values for existing people too? If so, In Marketo, you can block the field updates on the Person Source field or build a setup to write the data to a separate field and then restore the original value in the Person Source field when it's updated by the form-fills, but I think reaching out to Wistia support to check if they can help re-mapping the source field to a custom field of your choice rather than Marketo's Lead Source field would be a better option.