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You can only really deal with your direct employer.
You cannot register a lien because providing security services is not covered under the group of people that can register a lien on title.
You can take them to small claims court. That is the best route to go in my view.
What you should do is send them a letter by registered mail setting out what the owe you and providing them with a deadline to pay you. In the letter tell them that you will be commencing legal proceedings and will be seeking interest and your costs if they fail to comply.
You can then sue them in small claims court and self represented. If it gets that far and you find you need some help as you go through the process you can always post on the site again and I can assist you further.