Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to apologize to a neighbor for late night noise, use this factual base: state the date, place, property, noise, pet, or shared-space impact without arguing about the neighbor's character.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer cleanup, replacement, access, a schedule, or another practical remedy you can complete. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not threaten, shame the neighbor publicly, or involve the whole building in a private issue. Use property management, local authorities, or emergency services for genuine danger rather than confronting someone unsafely. You do not need the other person to agree with a boundary for you to follow it consistently.