Hmm, I think those permissions shouldn't be changed to "Read" for the users, because the Portal uses the userhome-folders (to store the favorites and deleted content for the user). So if you'd take away the "Full Control"-access from the users, there would probably be some issues sooner or later (I bet sooner^^).
So I guess you need to find another way to make this new requirement happen. Changing the permissions in the userhome around won't help you here.
You could create a new folder under "Documents", set the permissions there to "Read" for everyone and "Full Control" for the admins and then create a folder for each user under there (if the folder has the username as name, you can set the path there with "/Documents/<user.id>", too). Set the permissions for the folder to only the user and the admins and use that.
But that is a lot of effort, so I don't know if this won't be too much.
How many users are we talking here? Maybe create a script that can do that for you, I don't know (never done this before ^^).
I hope, the others here have some more ideas. 