When you disconnect from the network, you don't disconnect from the people, that you are already connected to and up/downloading to/from. You have to kill/pause those, if you want your Limewire running but not using bandwidth.

Alternatively you can close limewire and then restart it, then press disconnect right away. Or go to Tools-->Options-->Connections and remove the checkmark from 'Connect On Startup' (Note: you will have to go to File-->Connect everytime you want to connect if you do this)