Go to the menu System > Administration > Software Sources. There is a drop-down box labled "Download from:" where you can choose a mirror near you.
If this doesn't work as planned (might not since you have a "custom mirror") then you can edit the file
/etc/apt/sources.list by typing
sudo gedit /etc/apt/sources.list in a terminal and pressing enter, if you need help you can type
man sources.list in the terminal.
A source of the unwanted traffic might be from
security.ubuntu.com this is where the security updates comes from, they can be turned off, but it's not recommended!