In order to delete your account, please contact the Steam Customer Center.
However, before proceeding to delete your Steam account, make sure to read the warnings below:
1) Once you delete your Steam account you’ll not be able to retrieve it again.
2) After you delete your account, you cannot receive any refunds for the games you’ve purchased.
3) After deleting your account, you will not be able to use the remaining amount in your Steam Wallet.
4) You can not use any CD key that you’ve used in your deleted account.
5) Your profile will be deleted.
6) Posts from the Steam Community where your account was active will not be deleted.
7) You cannot use the same login ID after deleting your account.
If you've read the above warnings, please proceed to the Steam Customer Center and follow the steps to delete your account.