In the form's results table tick the checkboxes of the rows to remove (or the header checkbox to select all on the page), then click Delete Selected and confirm ("Are you sure you want to delete these results?"). The submissions are hidden from Results and from the submission count (they are soft-deleted, so Sprwt support can recover them if deleted by mistake). Deleting a whole form is a super-admin action (Settings > Delete Form in the form editor).