there's no option to really but as this is only your first post since you joined a few years ago and your account contains no images or personal details on show - i wouldnt really worry about it.
If you change your password to a random set of letters that you don't remember then your account is effectively deleted. Also, make sure that your email address is not set so that you can't later reset the password for your account. That should do it...
I'm not really sure why you'd want to do this, but I guess that's your prerogative.
Greets
Nefret