There is a fix coming up with some code I implemented that will suspend Mediaportal while games are running. I am waiting on the write permissions for the svn repository but when I can get a test version out this would hopefully help you with your problem.
Here is an example for call of duty 4
copy + paste the code in notepad and save as C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\runcod4.bat
Code:
C:\killprocess.exe mediaportal.exe
C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\iw3sp.exe
you could also try using /t
C:\killprocess.exe /t mediaportal.exe
this will attempt to close mediaportal nicely
I'm sure someone else could come up with something fancier that auto restarted mediaportal when you closed the game.