What do you tell people who say that communism won't work because “people will corrupt it and take advantage of the system the same way they do on capitalism”?

It is just a defeatist argument, basically the same as saying, "Well we all die anyway, so why worry about our health?". Sure, corruption is possible as with we saw happen with the Warsaw pact, but a socialist state being corrupt just simply means that is operating closer along the lines capitalism. By asking for capitalism, you are just giving up and settling for the worst.