I made sure that they understood why I felt using a translator for work that the students were supposed to do is plagiarism. This has to be the first step.
We have had some rather unpleasant situations recently with students using Google translate to write essays outside class. I agree that the policy has to be clear and that the implications have to be as well. Several instructors have moved to doing all essay writing in class.
On one of the very first days, I take the lyrics of a popular song that most students would know and paste it into a translator.