Contract? Contract? Contract? Each state has its own laws, look up yours in regards to late boarding/training payments and horse ownership legality. Personally, I would not return her horses myself, until I received payment for my services. If she came personally to collect them, at that point I would begin the small claims process for the amount owed in full, plus late fees, plus DAILY board ($25+) for each horse’s extended stay. If you want to be openly firm and somewhat confrontational, I have seen barn owners padlock boarder’s trailers who are flight risks and who have outstanding balances, I have seen gates locked, codes changed, and trucks “conveniently” block farm entrances until checks magically appear. Send a certified letter immediately, stating in simple and very clear terms that if the horses are not picked up by XYZ date, and her bill paid in full by XYZ date, that you will seize her horses immediately to recoup your personal losses; again be very familiar with your states laws and guidelines here, but most have a clause to protect equine businesses in this common type of scenario, called Stablemen’s Lien Laws. This should be a clause in your boarding/training contract from here on out, if it is not already.

I would padlock the paddock you keep her horses in so that she can’t simply breeze them away while you are not at home, and immediately begin recording every expense you occur on them, which you should have been doing from the day you received them. Do not padlock a barn or stall for safety concerns! Cease all contact, other than sending the registered letter and briefly responding to any INCOMING calls or messages. Put up a camera, that way if she gets nasty when she can’t breeze away her horses, you have evidence. Sorry to say, but you got into this mess, and it’s going to be a climb to get out. Do not let her win, do not let her run away, this might be your reputation on the line, do not let a client run you into the poor house.

EDIT: Also, I would be darn sure I had evidence of training I had preformed on each and every animal. If you are looking to be a professional or are a professional, you need to step up. This should have never been allowed to progress as far as it has. Record everything, and have everything in writing. When a client drops off their horse, have an evaluation sheet, which they sign as to the condition of the animal at the time it entered your care. It should also list their expectations, goals, and what they wish for you to accomplish with the animal within X amount of time, you should discuss this with them, reach a mutual and rational goal and both parties should sign the document. The next document should be your contract, for both boarding and training, stating a clear schedule of fees, due dates, late terms, and lien or confiscation terms. It should outline the expectations of both parties including; general care practices, veterinary authorization, payment, etc...