SISCO Saudi - Payment collection for recruitment services rendered in Saudi Arabia

We are facing a huge issue of payment collection from one of our clients and they have not been responding to us for two years now.
Below is the mentioned payment collection concern we are facing with one of our customers in Al Jubail, Saudi Arabia.
Customer Name: SISCO Saudi ( Specialised Industrial Services Company Limited)
Location: Al Jubail
Candidates placed by us is Gagan Swain and he was offered a salary of about USD **,*** and our billing amount is USD *,***. We have done multiple follow-ups for payments however every time they make one or the other excuse and we end up getting nothing. Mentally we are being harassed. I can share all the details of candidates, HR persons who are involved in it.
We want to file a defamation case against this company and charge them a hefty amount because we have suffered loads of losses because of them.
Will be looking forward to your help and solution at the earliest. Your kind reply will definitely help us.