Um, maybe it's part of the bandwidth that LW uses for messages and stuff (although you say you disabled UP and 15K is a little high...).
I know I may sound as if I think you're stupid (which I don't) but are you sure you click apply after making the changes in the Prefs?

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-