+1. Another argument for disabling form submissions. We had a form on our (non-Hubspot) site. When we stopped accepting submissions for this form, we deleted the form from our site, but some users still had the page open without refreshing for hours after we deleted the form, so they were still able to submit it.
The other suggestions for hiding fields and changing the submit button text may work but are obviously not ideal, and I assume won’t help much if the form is already loaded.