If you want to get very fine with this, you can create a new account for each place you visit. For example, if you must submit your e-mail address to Company X, create a new Google account for . Should you receive spam at this address, you have a reasonable idea of who's responsible. At that point you can complain or swear off that company in the future.

If you have a MobileMe account you can create up to five alias addresses. Messages sent to these aliases are forwarded to your primary MobileMe address. So, for example, when dealing with the famed Company X, create an alias for . After you've received Company X's activation e-mail, delete the alias. Any future messages sent to this account won't go through as it's now a dead address.