I've been asked the question often enough I felt it warranted a post on the matter. The short answer is account deletions or renaming are not done except for special circumstances. I'll try and answer some common questions on the matter below:
Why not perform deletions?
It can often break (or "ghost") active discussions, especially if the account started the threads.
I've never posted anything, can you remove?
At different intervals the user base is purged of orphaned accounts that have not been logged into or posted from. It will eventually be removed.
I don't like the name I picked signing up, can I change?
If the account is new, simply create a new one and abandon the old one. If the account is old and has a lot of posts, please see below.
I don't want to use the account any more and don't want to be tempted. Anything I can do?
Yes, you can enter a random password that you'd never remember. Change your email address to a bogus one as well so the account can never be recovered.
What are some special circumstances?
Top of the list would be using a recognizable alias that is used in vanilla/corporate life. Others are certainly considered, so PM me with your situation.