I've been teaching for 8 years now and I do have two regular 'free' riders - my sister and her best friend.
My sister 'pays' for her rides by exercising my horse, feeding when I'm busy, and riding as a lead rider when needed for less experienced riders. Her friend pays only for one-on-one lessons, but rides in group lessons for free. That is our personal deal that I offered her due to certain circumstances.
I would not advertise giving free lessons - because now you find yourself spending time teaching but not getting anything in return. I do give them on occasion as 'extras' - like birthday lessons, or when a lesson in actually full, but they ride as a tag-a-long extra.
For your situation now - I would explain to your boarders that you are no longer able to be so generous with your time, due to x, y, and z. You could offer a discounted rate to boarders, or even have one group lesson a week for free, that they are welcome to join on a week by week basis. But for private lessons - definitely charge.