Those webmail services all have an abuse@ email address. Hotmail would be
[email protected], Yahoo would have a
[email protected], etc. I know because I have used it and gotten replies. The only recourse there is that the user would have their account terminated.
ISP or corporate email accounts are better in that you can get the actual person in some trouble. For a corporate email account I would find their website then find a few contact email addresses (one for the hosts of their site and one for HR of the company), then ask if they are the correct people to contact if you have received some abusive emails using their domain name email account. An ISP email account (Comcast, AOL, etc) usually has an email address similar to the abuse@ that the webmail folks have. The kicker here is that the account is tied to real information about the user and thus action can be taken.