You can configure update manager to look for updates more unfrequently or even disable de automatic updates doing the following.
Open update manager.
In the lower left you will find the "Settings ..." Button click on it.
Go to the "Updates"tab, there you will find some check boxes and some combo boxes. The first combo box has the label "Automatically check for updates" and you can choose there the frequency for ubuntu to look for new updates.
You can even set it to "Never" and then you will have to check manually for updates. This is a non recommended configuration from the security point of view, but if your bandwidth is limited ...
Check also for applications like Ubuntu One, Dropbox or some other cloud sync applications which may consume your bandwidth.
Also some facebook.twiter.twenty clients may be looking for new posts and messages too much frequently.
Also check if you have an email client configured to download the emails.
Hope this helps
P.S.: Sorry for the long explanation but I can not upload pictures