Ninja excuses

"it's worthless so I'm gonna take it"
This is the attitude you often find when confronting a ninja, on RDF there was a veteran player as our tank who pressed need on everything he could (= ninja).
He even admitted that he only used those items for disenchanting, when I told him to roll fair he was a real **** about it and mocked me for "caring about blue items"

Bro he was almost BiS and a leader of a guild who cared enough about blue items to steal small dream shards on rdf lmao
And it's not like it was an absolute carry, I tanked as much as him while dealing 3x the damage

Just roll fair people

Did you report him?
If done with proper evidence, it's the easiest way to communicate this kind of person that they're mistaken.

Yes I sent a (partial) recording via an in game support ticket following the guide on forums, I got a very polite response but it didn't clarify much, just that it's going to be checked.

Players don't receive feedback on their reports beyond confirmation it was received (or that evidence is lacking, for cases when someone sends just their text and expect it to be taken at face value).
If the evidence followed the minimum guidelines and clearly shows what you claim, they will get a ban once the file is checked.