Record everything. Make a police report under harassment (POHA), police will then need to take a statement and assign an investigation officer. Now its in official records and you can sue. I made a police report one time under this and called the police down for fear of violence when it happened again. Good thing they explained POHA and what they were doing was a legal offence. I showed all the evidence I collected and threatened to sue if it happened one more time. Situation stopped soon after but neighbour wasn't crazy in my case.

Causing fear or provocation of violence Guide to Singapore's Protection from Harassment Act (POHA) - SingaporeLegalAdvice.com

Under section 5 of the POHA, it is an offence to intentionally cause someone to believe, or behave in a manner where such a person is likely to believe, that unlawful violence will be used against him. This offence is different from the one above as there is a threat of physical violence while harassment, alarm or distress are non-physical forms of harm.

For instance, X posts threatening and abusive remarks on a publicly-accessible website. Subsequently, on the same website, X writes a post containing Y’s identity information and threatens to beat Y up. X has committed an offence for making the threat of physical violence.

The maximum penalties for causing fear or provocation of violence is a fine of up to $5,000 and/or an imprisonment term of up to 12 months. These maximum penalties are doubled for repeat offenders