Linden Lab Official:How to Cancel an Account
You can always update your membership plan, including canceling it, by going to the Cancel Account page after logging in with your Second Life username and password. (This link is also shown by clicking the Account section on the left of your Dashboard.)
Once logged in, find the Cancel Account button at the bottom of the page. Your account will remain open until your billing review date, as account maintenance fees are paid in advance.
If you want to hang onto your name in case you want to come back, but don't want to pay the US$9.95 reactivation fee, simply downgrade to a Basic account. |}