If the free email accounts were accessed via un-encrypted HTTP then you could prevent HTTP uploads using a firewall that recognized that.
However, since most of these are encrypted (HTTPS), it can be difficult.
The best way I know how to do it is to just control the sites that people access via a content filtering solution. For example, we use Fortiguard and it can lock down all known email web-based email sites with a click of the mouse. An alternative would be websense but it is pricey.
Hope that helps.