I agree with Linney, and also Rick998. The disk cleanup only goes after "not-in-use" files. Worst case scenario, you can choose to only compress old files, and that will include the Temp. Personally, I feel that if a program that I install still is utilizing the Temp folder, it is either old, or not really a "good" program, as the temp folder is exactly that (only temporary files).
Rick998's idea about the logoff script is something that I have done in the past as well.