Unfortunately you’ve encountered inconsiderate neighbours, backed up by a landlord who doesn’t care and likewise his REA. There are some matters you have raised which are quite interesting, including the laundry on the balcony. Does that contravene another by-law? Are these tenants taking in children for reward and running an illegal child care centre, perhaps the council could investigate?
On issuing a notice to comply, this is no use unless it is properly followed up. The further warning letters are almost useless, they only indicate to these recalcitrant tenants that no-one is going to take any serious action to enforce it. You have the opportunity to apply for a penalty AND take the matter to the tribunal. If the notice was approved by the AGM it is up to the Managing Agent and the Executive Committee to follow it through to conclusion. Lean on them!
Hope you can get a speedy resolution to your difficulties.