The strategy to defend Keenum has been pretty consistent since his starts with the Texans. Get pressure on him consistently because he doesn't make teams pay. Keenum takes care of the ball. He doesn't force plays and would rather throw the ball away than throw to a covered receiver. That's fine to do most of the time, but when you do it all of the time, teams aren't afraid to send 5 guys over and over. Putting 5 guys on the line also makes it difficult to get the running game going. If they only send 3, get your hands up to deflect the screen passes.
One other note, it's also why Keenum has a much higher rating rolling out than in the pocket. It gives him better field vision for his stature and less batted balls. The problem is, you can't consistently roll out because it gives the defense more free shots at the QB.