See, the real challenge isn't so much keeping foreign life away from Earth. In many cases, that's exactly what we're searching for. We don't want it accidentally stuck to a Mars rover's treads, but we still want it. What we definitely don't want is a bunch of Earth life contaminating another planet or moon. This is for two reasons. One, we just shouldn't be spreading our own life to alien ecosystems, since that could disrupt a huge portion of organisms not used to terrestrial lifeforms such as bacteria. Two, we don't want to accidentally "find" life on another planet only to realize that it's our own.