Erase signatures from the Mac OS Catalina V10.15 Preview app forever

I've been using the Preview App to generate my passwords and to be able to sign virtually. But now what I want to do **is to erase them completely, so that nobody can access them.** For example, if I sell my Mac in a hurry, or if someone accesses my Mac, I want to make it impossible to access those signatures. I don't mean to press the X (cross) that appears after generating the signature; because I think that by pressing that, that signature must be saved somewhere. I don't know. What I'm saying is to **eliminate it in a more strict way, forever.** Is it possible to do this? How?