How can I remove a signature once it's saved in Preview.app? How can I delete a signature that I inserted using Preview.app? 

I can edit or delete all the text fields but the signature seems locked somehow.
A possibility is to try
File > Revert To > Browse all versions...  

though I guess it will depend on how far back that version history goes. I've been unable to discover just how far back that would be &amp; for how long it may be saved.