Go to settings --> forms --> edit form properties --> delete
We do not recommend deleting forms that are on user's accounts! Here are some alternatives:
Hide the form from users by marking it admin only - Here's how
Stop adding the form to users by deleting the trigger. Go to settings --> forms --> click the lightning bolt beside the form --> delete. The lightning bolt will no longer be purple.
If users have the form
There will be a popup explaining the consequences and number of impacted users. If the form is deleted, the information entered into the form will be lost.