Simply go to Account settings and click on the 'Terminate my Mailjet account' option.
Then you will be asked to confirm your choice, and after that, your account will be permanently deleted.
* Make sure you have backed up all your data before deleting your account, as all information will be permanently removed from our servers and there won’t be any recovery option.
** If you are on a custom subscription, this option will not be available for your account. Please contact your account manager or open a ticket with our support.