The in game method requires an ally code, which I don't have. We have a guy in our arena shard, that I reported a year or so ago. He seemed to disappear, but unclear if anything was done. He would...
You can find ally codes through googling swgoh.gg and their guild, but sounds like that might be hard.
There isn't any rules against sniping players, cause well it's arena and meant to be a free for all. Conversely, would you cry foul if you were in a shard mafia and was constantly getting him chained out of top spot? You can't have it one way and not the other.
Also also, if you're talking about squad arena, (which its 2025 so I'm hoping not,) you aren't losing crystals, just SSC. Let him waste his crystals.
Imitating a player is technically against the "Play Fair" standards of EA, but I don't know how they enforce EA standards vs TOS of the game. I view it more for trying to have authority over someone and less about confusing someone for PVP.
Unless something is said in game chat (discord is 3rd party) that is threatening, vulgar, against the TOS. I dont see it being harassment.