It sounds like the trainers are taking advantage, and if one horse is leaving for a race or show, they are bringing in a different horse.
I would have a description, with tattoo number of what horse belongs in what stall.
Tell them if they rotating 3 horses then they are paying stall board for 3 horses. not stall board for horse 1, and pasture board for horses 2 and 3. Stall board for all. If a stall is not assigned to a particular horse Put a Lock on it. If they remove the lock make it a 50 dollar damages fine. I would do the debit card/ pay pal or whatever. Also, it may be more time consuming, but if some boarders find the 15th instead of the 1st of the month easier for them to pay, you will have less hassle to collect the money . The last place I boarded at, I paid on the 2nd friday of the month, that was when I moved the horses in, so it was easier to pay on the second Friday. No set date, just the 2nd Friday of each month.