Nope, it's not Trust abuse. Anyone can leave whatever they want as feedback, especially if they are leaving as a warning for suspected thieves. The key is to make sure it isn't frivolous feedback resulting in the dilution of your own reputation.
Your continued posturing for "proof of scam" is a red herring.
Anyone can leave whatever they want as feedback
That's where the abuse comes in. They can ruin anyone's reputation without any reason.
especially if they are leaving as a warning for suspected thieves
Why call him "thief" if he is a "suspected thief" ?
What is the reason that you are suspecting him to be a thief.
Your continued posturing for "proof of scam" is a red herring.
That doesn't make any sense. Asking for a reason or proof that someone have done this thing in past is not a "red herring".