Looking back through some old threads about people receiving letters asking them to calm down their usage ages ago I see no mention of said department.
You may want to take a look at
http://www.broadbandreports.com and the ISPs who cap there. Various things being used from a simple automatic email right the way through to 'walled garden' solutions where users are redirected to a web page indicating that they have broken their allowance and are required to click a button indicating that they will take it easy for the rest of the month.
If you think that ntl lack the resources or wherewithall to implement either of these you're kidding yourself sir.
Ooo, edit, the personal kicker for me is OptimumOnline's solution, if you upload too much in one hit they send a new config file to your modem dropping the upload from 1Mbps to 150kbps until you either wait for it to be removed automagically or phone them up and promise to behave.