I want to prevent anyone from logging in to subsites

Hey guys,
I want to prevent anyone from logging in to subsites from their own mysite.domain.com/wp-login.php or /wp-admin.

Basically the only way i want people to login to their site is from my main site.

I think this would add another level of security for the users + it would get my site more page views.

Is it possible to 'turn off' login on the subsites + return 404 errors anytime someone tries to access /wp-admin /wp-login.php from a subsite.
OR redirect back to that subsite's homepage?