Lots of psychology and helpful "mentoring" techniques but, in all honestly, this just boils down to "write tests if you want to still have a job, tomorrow."
You can couch it in whatever terms you think are appropriate, harsh or soft, it doesn't matter. But the fact is, programmers are not paid to just throw together code & check it in -- they're paid to carefully put together code, then put together tests, then test their code, THEN check the whole thing in. (At least that's what it sounds like, from your description.)
Hence, if someone is going to refuse to do their job, explain to them that they can stay home, tomorrow, and you'll hire someone who WILL do the job.
Again, you can do all this gently, if you think that's necessary, but a lot of people just need a big hard slap of Life In The Real World, and you'd be doing them a favor by giving it to them.
Good luck.
