Blog users and avoid or prevent they log in

Hi,
I receive several users that are connected as subscribers in the website http://www.demosandpolis.com

I would like to prevent people to suscribe as I did not include any link or option to do that.

The only way people can suscribe is vía Aweber service, in the form that I have include in a widget.

How can I prevent other users to suscribe outside this Aweber service?

Thanks in advance,

Virginia