If you’ve registered there’s no way to delete your account. The account appears to ‘go away’ after not using it for a while, but this is not the way it should be.
Try to contact them to have your account removed manually. Use this link: http://help.kijiji.net/ca/knowledgebase.php?article=226 for various contact options, including a live chat. Tell them they should add an account removal option!
If you just put up an ad and did not register (taken from their FAQ):
We will send you an email notification once your Kijiji Ad is no longer on a prominent page. The email contains links to bump up your Ad and to buy other optional features. It also contains a link to delete your Ad from the site.
Should you no longer want to receive this email, click the link found at the bottom of the message to unsubscribe from our notification system.