How to remove your Steam account
Steam is very unclear about account removal. You should try to file a removal request using the contact form found here: http://store.steampowered.com/ssa_feedback since your account cannot officially be removed by request. On their support pages they state: ‘Only inactive accounts containing no games are removed. Accounts cannot be removed by request.’ However, one of our visitors pointed out that some removal requests are in fact granted, so it’s worth the try.
A new name can be registered if for some reason you need a new account. For more info on cleaning up your computer from all Steam-related data see: http://answers.yahoo.com/question/index?qid=20070913044038AAQrc8N
Then, the Subscriber agreement of Steam states that you can always cancel your account (but how?!). See: http://store.steampowered.com/subscriber_agreement/
Concluding: it seems nearly impossible to delete your account, since it can never be done by yourself, except for ‘removing all your games from your account first, hoping for the account itself to be removed’. Probably this will be done automatically since then your account will look obsolete.
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Steam using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Steam. Click here to find out how.