Once applied, the PDF sign signature can no longer be edited or deleted in Foxit Reader.
To delete a signature, you need Foxit PhantomPDF Standard/Business. After you've installed Foxit PhantomPDF, you may open the PDF document then go to Edit > Edit Object > Image, then right click on the PDF sign signature and click on Delete option in the pop-up context menu to remove the signature.
For more information about Foxit PhantomPDF, please visit https://www.foxitsoftware.com/pdf-editor/