I knew a guy who had a very common email address (like jsmith at popular provider), and it was pretty common for people to sign up to services using that address.

For well designed sites that require verification before the account is setup, just trash the message.

For other stuff, you may have to unsubscribe.

Worst case scenario for something like a dating sites or whatever, you can just reset the password, then login and delete/deactivate the account.