Sign-up forms can't be deleted but they can be deactivated.
To deactivate a Sign Up form follow these steps:
This step changes your list from a Public List to a Private List. A list has to be Public in order to have an active Sign Up link assigned to it.
Because the Sign Up link is still live (meaning someone can click on it) you'll want to edit the Sign Up form.
To Edit the Sign Up Form:
Click on "Contacts"
Put a check mark next to the Sign up form (the name of the Sign up form should now appear in red) and click Edit . Then change the text of the Sign Up form to say that this Sign Up form is no longer available (and provide them with an alternative way to reach you , if you want.)