A reasonable legal requirement should be that customers are able to unsubscribe using the same method used to subscribe and the process should not require more time and effort than the initial subscription.
A more general solution is to make the payment infrastructure allow me to ban a particular merchant. You can implement this by reissuing a debit card, but there's no reason not to make it seamless for individual merchants.
Another thing that helps if you don't want to fight someone who violates this and they require you to send a letter, that an e-mail also qualifies as legally binding. So, if they ask a letter to end a subscription, they must also accept an e-mail.
(IANAL)
Their 35/50Mbit upload speed says differently. I'm really looking forward to not having to call them again for discounts (since you otherwise pay more than new customers) because I can then actually leave them when the fiber is installed.
Good to know I just have to hang up on the sales guy.
Edit: according to member t0mas88, it is not just Portugal, or the Netherlands as mentioned nearby: it should be a European directive, not yet implemented by all Members. I guess that this should push heavily on the service providers for general compliance (as opposed to changing the options according to geolocation, as another member here revealed mentioning California).