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.
Note

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

Form Pre-fill does not work when embedding a form on your own pages. It only works on Marketo landing pages.
3. Select one of the fields and set Form Pre-fill to Disabled.
Tip

You can also disable form pre-fill at the landing page level or at the admin level.
4. Click Finish.
5. Click Approve and Close.
Nice job!