I can think in some possibilities to deal with this problem, but I don't know if they're the best policies.

1 - Ask for a statement of contribution (similarly to the ones we see in scientific papers). The students would need to lie in a document if they would want to carry others with their work;

2 - Perform individual evaluation about the project. You can do oral or a written exame about the project, based on the statement of contribution;

3 - Grade the students accordingly with the statement of contribution (not always possible);

4 - The most efficient method (with less work to the professor) : stop giving group projects.