not delete the history actually
just clear the "historys" in the upper part of the message box
but, the history is still here
just like you write something like
<me> I hate john
<tom>Yes me too, he sucks
and at this time, John is coming out from my back, so I can clear the words immediately, maybe like 'clear message sesson...etc' in ICQ lite
which clear the messages in the history part