Kevin Flahaut has put together a blog post on how to use jQuery to clear default field values when they are selected by the user filling out the form.
This is a feature that people have requested before, but isn't built into the plugin. You can accomplish it yourself with the available Gravity Forms tools and adding some jQuery to your theme.
Get all the details here:
http://www.gravityhelp.com/clearing-default-form-field-values-with-jquery/