Package: kmail
Version: 1.1.90 (KDE 1.93 Beta >= 20000813)
Severity: normal

1) Open compose window
 (I hope you have a signature automaticaly inserted)

2) Press Ctrl+Z
  Now signature will disappear. 
I think you register in undo LIFO even signature inserting.
Usually would be good to undo only things writed by user.