That's actually exactly the reason I created my xrestrict utility. Here is its github page. Look at the Building section to see how to build it, I'd also be happy to walk you through setup and basic usage if you're not familiar with building software from source on Ubuntu. I also wrote a blog post describing the utility in a bit more detail, and illustrating what it does. In short, once you build it, just run xrestrict -I and click on the monitor you want to use using your tablet stylus.