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”?

There's no simple answer. It's an argument based on a conception of universal 'human nature'. I reject that conception.

According to the Marxist world outlook, the only thing constant is change (dialectics is the study of change). People tend to accept this idea because it's obvious when they think about it, although it can be discomfiting. If it helps, you can use the example, "Can you step into the same river twice?"

What is less obvious (because liberalism hides them) are the implications. It can be helpful to focus on a 'moment', freezing time for long enough to analyse what you're looking at. But this is only an analytical trick.

If everything is always changing, then any explanation of anything is only valid for a specific period of time. Before that time, it was something else. After that time it will become something new. Everything is historically contingent.

This applies to 'human nature'. Of course, a liberal might say, 'well yes but the change with regard to human nature is that it didn't exist before humans and won't exist after humans' but that's silly and completely misunderstands things. For a start, it implies that human nature has been the same for 200,000+ years, which is frankly a ridiculous assumption.

The next question is, then: What affects human nature? What influences the idea and the practice of what it means to be human?

According to the Marxist world outlook, the object of study is not so much 'things' as it is (internal) relations and processes. (All this wraps around itself: understanding 'change' requires you to understand the idea of a 'process', for example.)

Human nature is internally related to other social processes. Humans cannot survive in the abstract and cannot survive as a species as individuals. Which means humans don't exist without society.

The organisation of that society tells you how humans meet their daily needs. Human society is organised to make shelter, harvest and cook food, educate and care for each other, and so on. Although the exact form of organisation changes, to reiterate, there are no humans except humans organised in society.

The way that society is organised for meeting human needs determines how humans interact with one another. Abstract factors that influence the internal relations of humans and society include: the mode of production, means of production, relations of production, forces of production, mental conceptions, technology, geography, urban/rural divisions, agricultural advancement, etc.

If all this is true, then human nature cannot be universal. It must change depending on the kind of society in which humans live. If human nature is not fixed, the fear that humans are naturally corrupt is misconceived.

It's as simple as this: in capitalism, if you play by the rules and are not corrupt and greedy, there's a high chance that you will become unemployed, homeless, and eventually starve to death. (You can break the rules and work cooperatively e.g. in a union, but you will be disciplined, attacked, slandered, and penalised for it.) Capitalism requires corruption and greed. It literally cannot exist otherwise, although it uses different labels for them, like 'competitive', 'efficient', 'lobbying'.

It doesn't have to be like this. And it will not be like this under socialism and certainly not like this under communism. Under socialism, greed and corruption will be less useful because essential needs will be met collectively, regardless of how competitive or efficient (or 'connected' to the right people) anybody is. Those who insist can be disciplined, attacked, slandered, and penalised for it, just like those who work collectively under capitalism.

At the same time, 'the ruling ideas of the epoch are the ideas of the ruling class'. This phrase will apply when the workers are the ruling class just as much as now, where the capitalists are the ruling class. In a society that consciously organises itself collectively, the ruling ideas (shared in culture, literature, film, water-cooler conversations, etc) will not be of greed and corruption because these practices will not bring the same advantages as they do under capitalism: the new social relations will begin to render them obsolete.

In sum, I politely tell the other person they don't know wtf they're talking about. Or I tell them that I'm uninterested in idealist or metaphysical world outlooks. If that would result in a 'debate' or argument, I probably just let them be. I won't learn much from anyone that doesn't understand or at least accept the premise of historical materialism and if they see it as an argument, as if the broad brush strokes are open to dispute, they aren't interested in learning from me. So I only say anything like this with those who are open to listening and learning.