ISPs deal with this in some legitimate ways like throttling (deprioritizing bittorrent packets so that they're first to drop when congestion occurs or policing the endpoints to a maximum throughput rate) and some not-so-legitimate ways (injecting connection reset packets to disrupt sessions).
Sounds like a strawman to me. No one (except perhaps the anti-NN folks, like yourself) has proposed that throttling excessive usage goes against the tenets of NN. What NN does argue, however, is that throttling *based on endpoint* is not kosher - mainly because it provides a strong negative incentive to customer quality.
From the FCC Commission Document ( http://www.fcc.gov/document/fc... ):
No Throttling: broadband providers may not impair or degrade lawful Internet traffic on the basis of content, applications, services, or non-harmful devices.
Don't confuse last-mile congestion issues (that you raise, and are legitimate) with throttling the interconnects. In your example, the BT excessive user should expect to hit monthly caps (which are not covered by NN) or overall throughput caps, especially during peak times. That's all (again referring to Commission Document) considered:
Reasonable Network Management: For the purposes of the rules, other than paid prioritization, an ISP may engage in reasonable network management. This recognizes the need of broadband providers to manage the technical and engineering aspects of their networks.