Everyone is talking about the legal approaches, or fine print of contracts, and so on. But if you want the real answer, like what actually works, then try to s

Everyone is talking about the legal approaches, or fine print of contracts, and so on. But if you want the real answer, like what actually works, then try to shame them in public or to people they care about. Like if the CEO posts on LinkedIn, post underneath saying "How do you reconcile this with not paying your freelancers?" Send an email with every single person at the company you have an email address for, ideally including the top people, copied, and ask them the same question, while asking if this is a common problem at the company, like are you guys all getting your paychecks on time, or is this just something you do with freelancers. If your client has case studies or a list of their client's logos on their website take a guess at who their contact would be at that client and reach out to them via email, copying your contact at the offending company as well, asking them if they've had any issues with financial misconduct at the offending company during their engagement with them, because you have. Clearly there are pros and cons to this approach, and your own reputation to think about, as well as fully burning the relationship and so on. But assuming you're OK with all that, this tactic, unlike the others suggested, is completely free and exremely fucking effective.