When a web visitor is known (cookied), Marketo forms will pre-fill fields with their information by default. If you want to turn this off, here's how to do it.

Form Pre-fill is enabled by default. Landing page level pre-fill settings and admin level pre-fill settings trump the form level setting:

Form < Landing Page < Admin

Go to Marketing Activities.
Select your form and click Edit Form. 

Form Pre-fill does not work when embedding a form on your own pages. It only works on Marketo landing pages.

Select one of the fields and set Form Pre-fill to Disabled.

You can also disable form pre-fill at the landing page level or at the admin level.

Click Finish.
Click Approve and Close.

Nice job!

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