I sent an email to customer support, where I stated I will use the right of withdrawal from the contract I had made with them, hoping they would remove my "debt". (I quoted the contract from their site, where it said that I had a right to withdraw from the contract within 14 days, without any reason.)
They claimed I had clicked "accept" to an option to disclaim all legal rights of withdrawal. (I don't remember having done so, but even if I had done so, I highly doubt it is a legal way of doing things.)
They further threatened me that if I didn't pay the debt, their lawyers would take legal actions.
I've concluded this is a scam, so obviously I won't pay them any money, no matter how much they threaten.