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.
Select one of the fields and set Form Pre-fill to Disabled.
Click Approve and Close.
Want to learn more about forms?