While I agree that ISPs will terminate accounts, the problem is with something like yahoo mail is that you can just open up a new email account (which is probably why them just add a number onto their last account name) as yahoo mail accounts are free. Most people who have yahoo, hotmail, aol, etc. email accounts are not using those companies as their Internet Service Provider, they're just signing up for free email accounts. Free email accounts are a dime a dozen and even if you shut down one account, or even get them barred from opening up another account with that provider, they can find a new free email account provider and have a new email account set up in less time than it took me to type this.
Now, true ISPs will definitely delete accounts if there is enough evidence to warrant it and may even cancel their internet account with them altogether, but you must have the IP and trace it back to find out the ISP. We do this when people have tried to attack our servers and ISPs will delete accounts quickly for that; however, if they are based in another country, it becomes much harder to get something done about it.
In any event, although I sympathize with your plight Atrocities (as I have enough email accounts that I get hundreds and hundreds of spam a day, not counting the spam that is autmatically deleted by the spam filters on Shrapnel's and our -mine and Richard's- personal server), I'm going to go ahead and lock this. I'll leave the thread up, but please don't continue to use Shrapnel's site to further this agenda.
Frankly, your actions, although understandable, probably won't give you the desired affect anyway because in all probability these people never pay attention to the email they are getting to these accounts, they're just using them to spam and are not their personal email accounts.