Ask Lemmygrad

This is far from a complete answer, but it might at least get past that initial objection, if they're willing to listen to you.

Under capitalism, the unscrupulous do not take advantage of the system, they take advantage of people who have little room to object because the system purposely denies them self-determination. A poor individual who tries to escape their exploitation risks homelessness and starvation, while the exploiter who fails is usually free to try again.

Under a system that protects everyone, however, the unscrupulous lack leverage over others, who can safely refuse to go along with their schemes. Without the imbalance of power, corruption is made inherently more difficult.

If you're prepared to also discuss prison abolition, etc., you might also add that, beyond that, societies already have mechanisms for protecting themselves in the form of laws and consequences for crimes. Each society's responses may be different, and need not be as inhumane as those we're familiar with now, and so on.