How do you deal with an extremely uncooperative shard member?

The very existence of "Shard" is the problem.
The person in question is essentially fighting against multiple enemies all on their own. Forming a collusion within a "Shard" to gang up on players outside the Shard is far worse.
As suggested in the statement below, the best solution would be to remove crystal rewards from Fleet Arena, just as they were removed from Character Arena.
With only GAC, collusion wouldn't be possible, and the administration could focus on managing cheats.