Actually, this is NOT an easy task. There is no option to turn it off, nor is there some option on the pages that determines whether a user may subscribe or not. You would basically have to remove any reference to this in the templates.
What I would do is create a plugin at "usersub_start":
PHP Code:
standard_error('Sorry, but Subscriptions are disabled.');
That basically throws an error anytime a user tries to do anything with subscriptions. Then I'd decide if I really wanted to go through and remove stuff from the templates. (I'd probably remove the lins in USERCP_SHELL and navbar and see if that was enough to satisfy me.)