[AprilPearl]

The way to solve picky eating is to be tough as you say and not put any more food in the bowl until all the old stuff has been taken. It may take a while, but hamsters will not let themselves starve and will get the hang of the fact that they need to take it all. Plus, you should only give about a tablespoon of food at a time as overfeeding encourages selective eating. So, you're on the right tracks having said all that, if he's only leaving one specific thing that's probably not too bad.