How do you delete you RuneScape account

RuneScape accounts cannot be deleted. In the past it used to be possible to delete an account, allowing e-mail address reuse, however the deletion request link did expire after a set period of time elapsed. 

It's no longer possible to delete an account. Jagex will not delete any accounts on request either, so you're out of luck there. If you'd like to disassociate your e-mail address from RuneScape, you'll have to remove it or (if you don't want to receive e-mails from RuneScape/Jagex) adjust your e-mail options accordingly from the RuneScape website. However, I don't believe it's possible to register another account with the same address.
If you want to create a new account you'd have to create a new e-mail address and account to associate with it if disassociating your e-mail address does not allow you to create a new account, or else retire from your account entirely. It's not known whether Jagex will implement a method to delete accounts.