Judging by the Court press release[1], it would seem that the unreasonable cost of filtering all P2P traffic for copyrighted material (yes, they actually asked for that) weighed heavily in the Belgian case. This would likely not be much of a factor when it comes to filtering a single domain, as mentioned in the UK case you linked to.

Still, the injunction by the EU court could well shift the balance even in cases where blocking is more technically feasible. Fingers crossed.

[1]: http://curia.europa.eu/jcms/upload/docs/application/pdf/2011...

I think that legally there is a large difference between them a) blocking one site for all users, and b) monitoring a users actions.

In theory you could do the former without ever recording which users attempted to reach the site in question.