First off, get off the release candidate onto the final release.
Secondly, there are a few ways of solving the problem. Probably the simplest is to block any outbound traffic on 80 and 443 (yes, there are other ports, but if you're not blocking by default it's a good start) and allow traffic to the proxy. Then simply tell people that they have to use the proxy. You could also install Squid in transparent mode and configure MLabs as your upstream proxy (and you'd still have to block 443).