Here is how you go about that.
Click on the 'Contact Us' link and ask the site admin to cancel your user name.
Please confirm in your request the user name and email address you signed up to PPRuNe with.
This helps ensure that the person making the request is the legitimate holder of the account.
Please also confirm whether you want all your postings on the site to be deleted.
If you choose to ask for this, then the postings are not recoverable once the action has been taken. Once it's done, it's done. If you do want your postings deleted but forget to ask at the time, or don't initially but then change your mind after your user account is deleted, then it involves a lot of manual deletion by the site admin, and will be a very low priority for action.
SD