If you are referring to tbbmeter it would not actually be able to show the wifi usage and an easy solution if it is wifi
1. Change the encryption key for the wireless network
2. Turn the wireless off.
If you have tbbmeter running, then right click to get the menu open, pick Statistics/Alarms and then select View Sites Visited.
This page tracks where data is being uploaded to or downloaded from. Double click the host names cell to see what the host name is e.g. if you see 188.8.131.52 listed that will resolve to forums.thinkbroadband.com (i.e. us).
This gives you an idea of where, but not which application is causing constant traffic. To see that close the Sites Visited window. and under the Test Tools menu, select Record TCP for 30 seconds. After 30 seconds you will be asked where you want to save a CSV file.
CSV is comma seperated spreadsheet that can be opened by excel or open office, and this will list the actual application that is doing the data transfers.