Step 1: Fill out the information below
There is no way to automatically deactivate your account. You have to send in a ticket with the following information:
- Account username (the username you use to login)
- Summoner name (the name that is displayed to other people when you play a match)
- The server you play on (NA server, EUW server, etc.)
- Creation date of account
- Location that you created the account (City and country)
- The email address you used to create your account
Step 2: Send this information to Player Support
Go here and then under "Please Choose a Request Type Below", click on "I have a question about League of Legends/Riot Games" (there is no better relevant option, unfortunately).
Note that you may be required to sign in.
For the Subject field, type in "Account Deactivation".
Under Category, pick "Account".
Fill out the rest of the fields accordingly, then submit your information in the "Description" textbox.
Now all you have to do is wait for your account to be deactivated. It will take up to 15 days for your account to get deactivated.