Is there a way to force apps to use internal memory only?

I have a Sony Xperia U, and my 8GB of memory are divided in three parts: 2GB phone memory, 4 GB internal memory (replacing the MicroSD slot) and 2GB that I haven't really figured out about. I don't want to mess with repartition. I just want to use each one as they must be used. The problem is: many apps are optimized to save many things in SD, but for this phone it is not really useful, as it has plenty of space in phone memory! What can I do to make automatic created app data use internal memory **only**, leaving SD (internal, in Xperia U's case) free for my own use? Edit: Thank you for your answers! John, my device is rooted, so, could you explain further about those permissions, please? Thanks in advance!