Sign up forms can't be deleted but they can be deactivated.
How to deactivate a sign up form
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.
How to edit the sign up form
Click the Forms tab.
The name of the sign up form will appear in red if it is not connected to a contact list. To edit the form, click on the form name. 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.)