Some arguments in favor:

• Their money was effectivly stolen, so it's not really "theirs".

• Selectivly punishing bad actors discourages such behavior.

Some arguments against:

• Deontolgy/rule utilitarianism says to not lie (perhaps with exceptions for extreme circumstances, which this isn't).

• It's illegal, and societies only function when people follow even the laws they don't like.

• The outside view says one could be mistaken about whether they're actually scammers. Maybe one made a bad business descision, and it's made them perceive the other party as a scammer, when in reality it was just poor communication. This is why we defer justice to the courts rather than trusting vigilantees.

Market resolves once I'm convinced one way or the other for the following question:

"Is it ethical for me to decide on my own to scam someone who I believe with high confidence to be a scammer?"