If you think an application stealing focus during your FPS is annoying, try
it when you're trying to admin a domain remotely.
As to your question -- Unfortunately, no, I know of no way to prevent apps
from stealing focus. As you have noted, even the PowerToy didn't really have
ultimate authority over the behavior. But I do have some ideas about what
should be done to the people who write applications which do steal focus, and
to the people who write operating systems that make focus stealing possible.
(Yeah, I know it would be hard to prevent. But still...)
There is absolutely NO reason good enough for an application of ANY kind to
steal focus. Period. Anyone who disagrees with me is WRONG. There is a system
tray for this sort of crap. Your application should not intrude itself
between me an the object of my interest at the moment. It's just like shoving
your face between two people who are having an intense conversation. Popping
up a a dialog under my cursor just as I'm clicking the damned mouse button is
a sure-fire way to make me think of driving nails through a baseball bat and
making a visit to your house! Do you hear me, programmers? Huh?
(wipes spittle off of chin)
Sorry about the rant. You do have my sympathy.