Request forms are available in Enterprise and Business accounts.
Request forms can be submitted by non-Wrike users with a link. These links can be invalidated at any time if you no longer want those they're shared with to submit that form.
Navigate to the request form builder for the relevant form.
In the left-hand panel un-check Enable public link.
Invalidating a link doesn't delete your request form. Internal users may still be able to access it, and you can create a new link at any time.