In Portugal the law makes it so you can cancel any service using the same means that you used to subscribe it, so if they support subscribing online, unsubscribing also has to be doable the same way; same goes for via phone, personal or whatnot. It makes sense, prevents service providers from making it too difficult to terminate a contract.
This should be the way for everything. I'm about to move and I need to cancel my power and my cable and I just want it to be as easy as logging into the system, selecting my last day of service, and that's it