First question before diving into JS: what does "signing up for a trial" really entail? In what way would directing these folks to a substitute Thank You page (and keeping them out of any trial flows) not suffice?
Hi Stanford - when someone signs up for a free trial it is a basic "contact us" form on a Marketo Landing Page that then sends them to a Marketo Thank You Landing Page stating "you will receive an email with a link to your trial instance within the next 24 hours and a representative will contact you to guide you through your trial experience".
The experience we'd like to see if they have a specific "company" then they either cannot submit the form, or if they can submit the form then have it go to a different landing page saying "please reach out to your rep for a trial".
The easiest way to do this is Add Choice under Form Settings » Settings » Thank You Page.
Blocking someone you have an existing, positive relationship with from at least thinking they submitted the form to you seems like a bad UX.
I am, as you may know, the world's most prolific user of JS to enhance the Marketo forms experience! But this seems like something that may not cross the line into code-land.
To add to Sanford's suggestion...
When routing the form submit to your representatives, you can add a choice in the smart campaign that says If email contains "@yourcompetitir.com" then remove from flow, or take whatever other desired action you want from there.That way your reps aren't bogged down with filtering competitors that they don't want to call.