Whenever you get abusive email, you should do the following:
1. Create a new email, including their entire email, INCLUDING ALL HEADERS. For example, in Eudora, click on the "Blah blah blah" button, to see the complete email. In Netscape, highlight the email & go to View -> Headers -> All. The headers are needed by the ISP (Internet Service Provider) to verify whom it came from. In Yahoo, view the email, clock on "More", then click on "View Full Header".
2. Send it to the person who sent it to you, with the words: "Don't send me any more email. EVER! I added you to my idiot filter". Add their email address to your filters, sending their email to your Trash (or simply watch for them to send you a reply). (In Eudora, go to Tools -> Filters. In Netscape, Edit -> Message Filters.) Bcc: their ISP. For example, if it came from AOL, Bcc: firstname.lastname@example.org. If there is no "abuse" address, send it to postmaster@ their ISP. Use http://whois.arin.net/ui/ or a similar site to find out their real ISP, in case they have disguised it. You enter the IP addresses (nnn.nnn.nnn.nnn) in the first field, and the last line of the response should contain their real ISP.
3. If you aren't sure who their ISP is: Look through their email headers for IP addresses. These are usually of the form a.b.c.d, where a, b, c, & d are numbers between 1 and 255. It might also be a large number without any periods (a.b.c.d = a*(2**24) + b*(2**16) + c*(2**8) + d, where "2**24 = 2 to the 24th power and a single "*" means "multiplied by"). Enter each IP address into "the address digger" (the first field) at ARIN http://whois.arin.net/ui/ and wait for the report to finish. The last site reported is the one you should complain to. E.g. if it is xyz.com, complain to email@example.com and firstname.lastname@example.org. Do this for every IP address in their headers. Some may be bogus, and may not supply an ISP. Ignore those.
4. If the email violates the law or advocates breaking the law (e.g. contains a threat), send a separate email to their ISP asking them to cancel the account, due to violating their Acceptable Use Policy (AUP). Use "Threat from our subscriber" or something similar as your Subject.
5. Look in your Trash every day after downloading your email, to see what the filters caught. Any email there represents someone whom you have told to stop sending you email, which is a clear violation of their AUP. Copy that entire email (INCLUDING ALL HEADERS) into a new email, and send it to their ISP with the Subject "Unsolicited email from your subscriber". In the email, say "Please cancel this account! I asked them to stop sending me email, but they persist." If they have replied to your request to stop sending you email, say "Please cancel this account, and don't give them another one! As you can see, I asked them to stop sending me email, but they persist." Many mountain bikers are stupid enough to do just that. They just can't help being defiant.
6. Post all hate mail, complete with all headers, in the relevant newsgroups (e.g. alt.mountain-bike (https://groups.google.com/forum/?hl=en#!forum/alt.mountain-bike), if they are a mountain biker), so that their behavior is publicly exposed. That allows their peers to "have a little talk with them" about making mountain bikers look badů. Use a Subject like "Typical Hate Mail from a Typical Mountain Biker" or something similar, so that it gets noticed.
In many cases, they will lose their account. They don't like that! It means that they will have to try to find another ISP, and possibly pay more for the account! This process will ensure that you never get more than one or two emails from any one person (email account, actually; some will get another account and try it again).