they're probably eligible for membership in Freelancers' Union (www.freelancersunion.org) who is currently involved in campaigns through their PAC to get legislation passed at both the state and federal level to protect freelancers and contract employees from deadbeat clients from this very type of corruption and graft. They also offer a contract generator for use by people who are not legally-savvy in order to insure that they have an enforceable contract. I'd recommend them, if a critical mass of independent truckers joined them or any such organization, they'd wield a great deal of clout because you'd have a unified means of fighting back without surrendering your independence.
Truckers may not seem like their target audience but they aim to represent all types of freelance and contract workers because a broad-labor base and many hands makes the fight for all of us working as freelancers (currently 1 in 3 employees nationwide, FU has currently over 170K members) easier. They also maintain a list of deadbeat clients for the benefit of their membership...presence on the list is an effective form of public shaming; they have no problem doing things like reading it out loud to the NY General Assembly or mailing it to the NY Times. (They're NYC-based but a national organization.)
Also, membership is free (They're an NPO rather than a labor-organization, they run off donations and grants.) and includes a fairly-large number of free benefits. (Member-to-member discounts, corporate discounts (hotels, accountancy, gym memberships, drug-stores, tax-processors, travel, workspace and 100s of other things), job-listings (mostly for freelance white-collar work, as those individuals currently make up the bulk of membership)) In addition, they offer pay-for-use benefits such as life insurance, health insurance, disability insurance, retirement services, etc.
They do a lot of advocacy and targeted campaigning so if you have documentable violations and people willing to go on the record, you should also email their advocacy team. The last thing any habitual violator wants is sunlight.
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