I am having the same problems and about to go down the same route. I was due to travel from GRU-LIM in Business class last August (2018) but due to a job change I cancelled and flew from London instead (on IB),
The fare rules stated a $200 USD penalty for cancellation so they owe me about $450 USD. Have made many calls but same old cr*p. Seems the problem initially was that I made the booking using my Mastercard which was on an Emirates NBD account (as I lived in Dubai). Seems Latam couldn't figure out how to/or process didn't work to refund me. So in Jan they took my UK bank details and I hoped that would solve it. Of course they said it would take 20 business days and of course at the end of 20 days no refund. It's infuriating as when you ring up they just close your case (Afterwrards) even though they have not resolved. You speak to supervisors who tell you the refund team is back office and no way to speak with them. It's now been 7 months and was really hoping to avoid the legal route but if this last case which I have opened does not work then seems there is little choice.
Really bad from Latam whom I actually enjoy flying.