Public forms can't be deleted since they are your pipe's start form.
What you can do, however, is disable the public form feature so that external users will no longer be able to access it using the public URL.
1. The first step to disable the public form is to click on the Share button, located in the header of the pipe.
2. Then, just click the Public button. When the key is green, the form link is available to everyone. When it is gray, the form can only be filled inside the pipe (start form).
3. That's it! Your form is no longer public!