What about websites simply getting there own back. Redirecting users to a page with the information on it to describe what the end users isp is doing.
Where the redirected page looks something like an IE6 warning going something like this.
Dear ...... We notice that the internet connection you are using is BT based. We have some issues with this because they do not provide enough bandwidth to deliver content from our site to its end users (that would be you) etc.. etc.. We would recommend that you switch to one of the following isp as they do not traffic limit content providers ..
Talk talk, BeThere, O2 .... etc.. etc... Great chance for the web sites to actually extort money from the other isp's to appear on the recommended list which is really getting their own back on the isp's but probably not recommended :)
I for one say to them let them try and see what happens. When things stop working for the end user. they are going to speak with the nearest techie they know and that techie is going to recommend a different isp anyway.