Well, he wouldn't lose his account for posting one troll or spam, but yes, ISPs have to stop their customers from persistent abuse or they'll get the IDP (Internet Death Penalty). Anarchy works! No system administrator can be forced to accept messages from a spam haven or any ISP that isn't cooperative about dealing with abusers. Giant corporations have much more to lose than small local ISPs and they are not all interested in risking getting their entire clientele barred from the net (or even small parts of the net) to protect the "rights" of one spammer or troll. They could lose a lot more than the $20 a month they get from him....
The usual procedure is to send an email to the sysadmin, who will issue a warning on the first complaint -- if it's a forum like this one (or an irc channel or a usenet group) he'll just tell the troll not to post there any more or he'll lose his account. That's usually sufficient, but if the troll persists he'll lose his account there, or if the sysadmin is uncooperative his whole ISP will have to be banned from Bladeforums. I don't think we've ever had to do that here, but it happens. That would mean any customers of that ISP that want to go to Bladeforums and no doubt other parts of the net too will have to change ISPs -- getting banned from Bladeforums alone probably wouldn't lose an ISP much business, but when a sysadmin tolerates abusers they get the system banned from one part of the net after another.... Spam havens can make money for a while because they charge the spammers a lot more than $20 a month, but they soon find they can't even get a local connection so they don't have any net access at all.
Occasionally there's a problem because you can't get hold of a sysadmin right away. I once had to ban an entire large university from an IRC channel for two days before the sysadmin finally responded and dealt with the problem -- just one troll, but she got all her friends to let her use their accounts and/or jump into the flame war on her behalf and every time I banned her she'd pop up with a new identity every few minutes. Fortunately we didn't have any other regulars from that university.... Then she moved to accounts at another college nearby (apparently she had friends there too) but I got hold of the sysadmin there right away and that was the end of that.
Take a look at the "Terms of Use" at your ISP....