You can delete it manually in prefs.js file, from about:config or, without K-Meleon loaded, editting prefs.js file in your profile.
This is sessions structure saved in prefs.js:
kmeleon.plugins.sessions2.<SessionName>.count
=> Number of KM windows open in session <SessionName>
kmeleon.plugins.sessions2.<SessionName>.windowN
=> Window N possition (where N is a integer)
kmeleon.plugins.sessions2.<SessionName>.windowN.count
=> Number of tabs in window N
kmeleon.plugins.sessions2.<SessionName>.windowN.tabN
=> Web page contained in tab N in window N
K-Meleon in Spanish