Subscription type plugin

I am looking for a plugin that will allow me to have users subscribe to my blog in order to use it. The content will be viewable to people who are logged in.

I downloaded and tested Members Only and I like that. It will protect my posts (i am not having any pages)

I am now looking for a plugin that will make sure there is only one person using his account. I do not want John to give his username and password to Jane doe. Any ideas?

To give an idea of what I am doing…

I am a blackberry theme developer, and I am making a site for subscription use only. when they subscribe/register, they will be able to login and download my themes for free. However, if they are not registered, they can not see anything or read any posts. This website is all mobile for all blackberry devices.

Also, is there a way to block users from looking on the site through a PC? I want this website to be STRICTLY mobile.

Thanks for your ANY help!!!

MG