It's not just the prefill that you want to ignore here, it's also the whole munchkin tracking. No need to associate the navigation that might be done from a laptop in the Van to the various people visiting it.
You will need to remove the munchkin cookie from the LP and also from the form. It will take adding a couple of lines of JS in the LP template as explained in this post: Re: Non-trackable landing page is still identifying me . You will have to pay attention to place the 2 code wisely, one at the end of the <head> and the other (the code provided by Sanford) at the end of the <body>.
You can also vote for this idea which is about making it easily: Create true "Untracked" landing pages