Just login to your website (I suppose it's a WordPress website) and deactivate the specific forms on the exact pages where they appear. Deleting the non-hubspot forms from the hubspot dashboard might not completely solve the problem, as long as you still have the integration active/intact and these forms live on certain pages where your site visitors can engage with them.
Thank you so much for your reply! Unfortunately, it's not that simple as we don't use Wordpress in this way. We still need these specific forms to work but just not import into HubSpot.