Penalty Box!
Here's another quick fix to stop students from speaking their native language...if I catch them, their name goes up on the board. I then add checkmarks for each additional time I catch them speaking their L1. At the end of class they have to come up to the front and pick from my "box" a mystery topic. They need to speak for 1 minute for each checkmark they have next to their name. If they stall, because they have nothing to say, I fire questions at them or have the class ask them questoins to promote discussion (based on the topic). If they have more than 5 minutes, they have to write a paragraph on their topic.