Ouch that hurts. But it doesn’t sound like you’ve been banned for life at least.

It’s difficult to give any advice here as we are still in the dark about a lot of details (that you probably don’t have either and will struggle to get unless the TA provides them). It was never clear to me whether the TA went into your account and booked flights for you using your avios and paying any fees. Or did they book flights for other random clients that you don’t know. Or did the TA just transfer the avios out into some master account and do god knows what. The first option would likely be ok and not controversial. The other two would be red flags and against T&Cs.

Unless you can get some some major game-changing further evidence then I guess you probably have to accept this. This evidence would have to come from the TA, who may be reluctant to supply it if there’s something shady going on. Maybe you could try to go to the higher ups in the organisation to get some answers.