BITS starts after disabling

Well it may not necessary be Windows Update exactly since BITS can be used to transfer data by any process or even a user, and is also how all modern apps transfer data in the background (among many other services and processes).  So yes, Windows Update uses BITS to transfer data, but so does dozens of other processes.  So just seeing BITS being used doesn't mean Windows Update is what is using it (it doesn't mean it's not either!)  But it doesn't matter in your case, all you care about is the fact that it's impacting you, which is understandable.

Your description actually helps quite a bit.  You see, BITS is supposed to test the amount of network traffic at your network car and use only the idle portion of the network bandwidth. But according to Microsoft:

"This can be an issue if the client has a fast network adapter but is connected to the network with a slow speed because BITS will compete for the full bandwidth instead of using only the available bandwidth on the slow link; BITS has no visibility of the network traffic beyond the client.

In other words, it can't tell that your bandwidth in your rural location is so limited.  The recommend solution is to use a system policy configuration to block it from using your bandwidth (as outlined here).

The Solution

Here's how to totally block BITS, limit it, or both.

1. Open Registry Editor and go to this location:
   
   HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\BITS
   
   
   
2. Right-click a blank spot and choose New > DWORD Value and name it as shown below.  Repeat for all other values shown.
   
   

This will block BITS completely between the hours shown, and outside those hours even then it will be restricted to 2 KBPS.  Of course you can completely customize this too if you want.

PS. If you have Windows Pro edition, you can also set these using a the graphical tool (it does the exact same thing and creates these registry values automatically):