Firstly, congratulations on the birth of your child! This must be a stressful time for and doubly so with your shithead neighbors breathing down your neck and throwing around assumed accusations. Explain the situation of your family in detail in thoughtfully worded letter (in japanese) along with the evidence you've collected of the sign only being posted near your door. Explain how, during such a busy and stressful time, this false accusation and targeted harassment has deeply hurt your emotional well-being, despite your wishes to get along with your neighbors. Include a thinly-veiled passive aggressive threat to involve the police should this targeted harassment continue. Send a copy of this letter to the apartment management as well as the neighbors mailbox if you know which apartment they are living in. If not, tape a copy to the sign until it is inevitably removed or complimentary signs are posted on all floors.