If you recieve a spam e-mail it will give you an e-mail address but thats usually not where it came from. You need to turn on Full Headers to see where it comes from. Check your e-mail services FAQ/Help for information on how to do this but if you use Hotmail go to Options Click Mail Display Settings and then where it says Message Headers select Full. Now return to your inbox and open the spam e-mail. It should look something like this:

notice the basic headers say the e-mail is from swbell.net but its actually from prodigy.net. Click the forward button and forward the e-mail to abuse or spam @isp.com. For example if it comes from hotmail the address to send it to is
abuse@hotmail.com. In this case its
abuse@prodigy.net. The ISP will then deal with the user and send you an automated reply to let you know they have dealt with it. You should check the ISP's homepage for more information about their Abuse/Spam Reporting services.
Tips on Avoiding Spam
When posting your e-mail address on your website or on a newsgroup or usenet make sure you forge your e-mail. For example if your e-mail is
name@isp.com make it name[nospam]@isp.com and then spammers won't be able to pick up your address and people who actually want to e-mail you can remove the [nospam] bit.
Another tip is not to use your real address but instead use redirects. For example when you sign up for a new service usually you have to confirm your registration to prove thats your e-mail address or to send through a code/password. If you create a redirected e-mail address then you can give them this redirector, read the e-mail, delete the alias and you get no more mail from them. This can be used on forums too. A good service is CJB.net. Even if you don't have a website you can sign up for their URL Redirection Service and use it for the free mail alias's it provides. For example if you created fakealias.cjb.net and your real address was
realaddress@realdomain.com if you have Mail Forwarding switched on you can
put-anything-here@fakealias.cjb.net and it will still go to your real address. Say one day you wanted to sign up @ Bravenet and you said your address was
bravenet@fakealias.cjb.net after you've confirmed it, if Bravenet sent you spam you could go to the mail alias options and create the
bravenet@fakealias.cjb.net and then point it at
where-the-spam-comes-from@bravenet.com and everytime they try to send you spam they will get it straight back!
HOPE THIS HELPS