Kerbals can play in sandbox mode and have access to an infinite supply of fuel, missiles, energy, and a debug menu. Not to mention that the Kerbol system has planets vastly larger than those in Spore (which are about Gilly-sized, I believe), which means the Grox may simply not pack enough delta-v into their ships to do much aside from some minor orbit adjustments. Kerbals just have to spam battleships into orbit until they have enough to handle the Grox with sheer numbers. Plus, Kerbals are physically superior, capable of falling from orbit and surviving (provided that they land on their heads) and are capable of lifting heavy spacecraft withough difficulty (Seriously. EVA a Kerbal, ragdoll him, land something heavy on him, and, in my experience, he should be able to lift the spacecraft when he stands.) And even if the Grox do take over Kerbin, Kerbals just have to install Extraplanetary Launchpads... Kerbals win. Hands down.