Sitecore XP does not support form data modification out of the box. Both Web Forms For Marketers (WFFM) module, and Sitecore Forms application do not store any data about the visitors, except contact ID that refers to the database. There is no special API to delete specific form submissions from the database. Users can add such features by themselves, for example, removing submitted form entries within a certain date range.
Sitecore will consider form deletion functionality in future Sitecore XP releases.