There is a question over at meta.stackoverflow that can answer this for you.
Bear in mind (as Jeff says in the comments) you'll need to do this for every site you have an account on.
I believe this deletes the account, but not the posts. You've licensed them to the site under CC-Wiki. Your username will be obscured.
If your account has never voted or posted a question or answer you
should see a self-delete option on your user page.
However, if your account has voted or posted, do the following:
Edit the "about me" field in your profile to say "please delete me" so we can confirm your ownership of the account.
Obtain your user page URL or numeric user ID. This is linked from the top of every page of the site when you are logged in.
Email the support address linked at the bottom of every web page on the site. Where it says "contact us", click that. In your
email, you MUST include your user page URL or user ID so we can
find your account.
Also of note, we automatically cull old inactive accounts as