Really that's a workplace psychology question, not a technical question, so as a techie I'm no expert. If someone started checking crap into an open-source project I was maintainer of, I'd revert the commit for a "first offence", withdraw write access (in favour of submit-patches-via-intermediary) on a "second offence". In the workplace, though, with people you have to chat to round the watercooler, it's probably better to be less hard-nosed in the pursuit of quality.

One way to be hard-nosed without seeming so, is to invent "staging" and "stable" areas, where people can check in testless code to "staging", but it needs tests before it's promoted to the "stable" branch where releases are made.

Peter