You can cancel the subscription on the JustFollow page. Money for unused days is refunded after an email to legal@suppy.org.
The subscription doesn't renew by itself, so if you just want it to end, you don't need to do anything: it turns off 30 days after payment.
The subscription turns off immediately: the cape, the coloured nick and the higher limits go away.
Cancelling on the site doesn't refund anything by itself. To get money back for the remaining days, write to legal@suppy.org. In the email, give your account's nick and say that you are cancelling JustFollow.
A block for serious violations ends the subscription. You can get money back for the unused days the same way, by writing to legal@suppy.org.
The full refund procedure is in section 7 of the offer.