• Click the Contacts tab
• Click on the name of the contact list that is connected to the the sign up form
• Click More Actions and select Edit List from the dropdown.
• You will get an Edit Email List pop-up box.
• Uncheck Allow people to sign up for this list box and click Save.

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.)

Or remove the sign up link or form from your website, Facebook, where ever you added it.

If you change your mind and want to allow people to sign up for your list click on the Contacts tab. Then select the name of your list and select Edit List from the More Actions dropdown.  Then recheck the box that says "Allow people to sign up for this list box" and click Save.