[JamesBarney]

Are you running into more design issues ie they didn't fully understand how to solve the problem so they are just throwing code at it? Or are you running more into code issues where the code is just sloppy?

For the first problem I would consider that after you give them a problem and they solve it have them white board it for you. Ask questions about how they handle certain edge cases or problems. If they haven't have them go back and figure it out.

For the problem with sloppy code have code reviews every few days. Just keep on asking questions about special cases, and giving advice on how to better structure code.