At the end of the day you just don’t want to do these things with Javascript (or form validations) on your website. It’s trivial to bypass, as you’re experiencing.
The better approach is to do this on “the backend” somewhere that folks can’t see or modify. Make (Integromat) works really well as a filter for this. The problem is that then you aren’t using Webflow’s native form submission tool. You’ll have to use a free Make (Integromat) account, and dump those results to a free Airtable account instead.
Here’s a walk through I posted on the forum not too long ago:
Also, you may want to think about a different approach when filtering emails. You will never catch every free email service that exists - it’s not possible.
What you can do instead is filter for only the business emails you’re looking for. Not sure if that’s viable for you, but something to consider.
Hope that helps!