Hello Jiamin,
So sorry to hear about what happened to you. Right now you need to pull yourself together and not be afraid. They are using harrassment and scaring tactics to make you pay them. Dont fall for their trap and report to the police. You didnt loan from them anything so they wont want to waste money on people to come and splash paint. Just ignore and block all their calls and messages. If possible just change yr num. Hopefully that will at least give you some peace.
Be strong and remember you are not alone. We are all here to support you through this. You need to be emotionally and memtally strong to fight against them. Explain to your mum and whatever she say just keep quiet and apologise. Dont argue with her cause at this moment you need her support more than ever. She is angry and upset, she'll say things she dont mean so try to be apologetic.
Make a police report immediately, change your number, if you provide your NOK number inform them about the situation, if you provide your workplace number inform them too. If you feel there's a need to install CCTV just go ahead. Tell you mum if she see any unknown people loitering around just call the police.
Be strong, be safe and dont be afraid. Jiayou, you can do this!