Because to remove an account in the system Steam is not available, use function lock. This is done from the profile menu on the website.
To lock you need to have access to your account and mailbox, which was specified by you during registration. Block account is done by administrators in case of revealing of infringements of conditions of use of the system. The reason for the account deletion may be the fact sales accounts, phishing attacks, stealing other people's accounts, joint use by several persons, the identification of violations during the registration of gifts, the hacking and piracy, fraud with Bank cards and so on. For more information about terms of use please read the relevant paragraph of the rules of use.
If your account is not blocked for any reason, please contact one of the technical support for the provision of information on the topic. After blocking you will be able to purchase licenses for computer games and other services offered by the service, but your accounting record will remain in the database Steam.
If you are just not going to continue to use the services provided by the service Steam, just leave the account record without changes. It is possible that in the future you'll need it.
Don't sell your account because it is prohibited by terms of service and also account blocking can lead to other consequences in accordance with the rules.
You can also purchase licenses for other users, using the account record, but a joint use of the account by two or more persons is prohibited and may also entail some negative consequences.