If moderation is something that BT is not able to do themselves, an alternative is to carry out a last 50 posts audit of a random sample of posters, perhaps biased to the most frequent posters. You could audit the last 50 posts made by a poster and categorise them as either acceptable or unacceptable. If you prefer, you could use more informative categories. I was going to have a go at doing this myself this weekend and started with my own posts using the following 4 category system of:
Informative: useful information that other posters can make use of
Acceptable: includes comments, opinions, queries
Contentious; comments that some people might take umbrage at.
Offensive: name calling, rants, and obviously offensive behaviour.
For myself, I decided my balance was 5/40/5/0. It might be worth posters self auditing themselves in this fashion but ultimately it would be for BT to carry these out unannounced.
I would have thought this would take BT less time than moderating every single post and anyone who breaches your limits can then be blocked as you wish.