Jim,
You mentioned a complaint by the neighbor to the board or management of the Co-op, but you do not mention what action the board took based on that complaint. Did they send you a cease and desist letter? What action did they take? If you were not asked to cease and desist from excessively flushing the toilet, it seems to me that there is nothing actionable. Neighbors in condos and co-ops make complaints all the time and nothing happens with them. I only remember one cease and desist letter at our condo and it was with a nuisance dog which killed another unit owner's dog. The owner of the killer dog was sent a letter telling him to remove the nuisance animal from the property. I also remember complaints over a tenant neighbor who my neighbors claimed was beating up his girlfriend and also abusing his dog (throwing it off a deck, supposedly). The board took a different approach and sent a letter to the owner of the condo and told him his tenant was being complained about and tell the dude to get his act together. I have seen this guy but he lives in a unit not so close to mine so I don't know what is going on at that condo. But there were multiple complaints about him.