A Signature Field allows you to sign with your finger or stylus on a touch-enabled device or with a mouse on a non-touch device. The captured signature is stored along with the GPS coordinates of where the signature was captured (which can be displayed on a map by clicking View Signature Info). You can also place a transparent signature field over an image to annotate the image.
/* prevent change if previously signed */
IF (OLD.MySignatureField IS NOT NULL) THEN
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Cannot change if already signed.';
A more complex trigger(s) could, for example, check other tables in the database and disallow changes to itself if other related records were signed. ParaSQL can provide consulting assistance (at our normal hourly rates) if you need assistance with coding these types of triggers.