There's no "delete" button, but if you really need to delete your account, you can send an email to email@example.com.
I'd be interested to know why you want to delete it? It's free, and doesn't generate spam after all...
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the reason you want to delete old unused accounts is for online security. unless you use a different password for litterally everything
Just go to your settings page and turn off all your Email Alerts, also disable your Private RSS Feeds and in two days you will never be reminded of Instructables again.