How do I get him to stop harassing me

So I divorced my husband of 16.5 years. We have 5 children together and I have one from someone else. I’ve been having a tough time dealing with the ex and need help or advice. He was to begin paying child support in December. He claims he has been paying into a limbo account, but alerted me when he had a threating letter from the friend of the court (foc) . According to the custody agreement, he keeps the oldest full time and I keep the other 4 kids 5 days a week, he 2 days a week. Initially child support was set at $1, 487, but I asked for it to be cut to $900. I have not seen this money at all. He recently came to me asking if I could take him off child support. He also wanted to claim 2 more kids on taxes because he claims the oldest. Not only did I already file, access to the divorce decree, I claim the kids because I’ve been taking care of them. He’s been buying new clothes and going to the gym. My kids don’t have beds at his apartment but have 2 flat screen TV, ps4 ps5 a computer. He wants me to take him off the child support because he can’t afford socks (his words) for the kids. I faxed the child support order and he’s been texting me daily. He blames me for him losing out on money. He told the kids to call me a thief because he’s paying so much in child support. He said I just bad mouthing him to the kids.

I really don’t know what to do? I can’t get him to stop harassing me