I worked at various gyms and sports clubs when I was younger, and had this exact situation arise many times.
There is room for discretion--all members should be served well, according to their differing needs.
When I saw someone who who seemed like they were hogging, I'd look at how much time they had left. I never would have interrupted at 54 minutes, I would have explained to the 2 guys waiting that I was going to give him until 60 because he would likely finish by himself. Same with times of, say, close to 40 minutes, which it seemed like people often chose, IIRC.
It wasn't just on the treadmills, either. Everybody understood the benefit of this system, and it was up to me as an employee to mediate between competing interests, in a courteous and calm way. It's really not too hard to bring things down to a simmer for 5 minutes, unless you are a maniac.
After letting someone finish in this fashion, we would ask them what happened, to better understand the situation.
With a half-hour limit on a piece of equipment, people will often hop on, and if there is nobody waiting at the time they finish, they will "start another 30-minute segment" without interruption.
That is certainly a fair way to characterize the situation. Consider this: what if, instead of continuing without interruption so that their clock now reads 54 minutes, they reset the machine so that the clock read 24 minutes instead?
If all the mills were full and someone wanted to use them, there should either be a signup list, or they should take the responsibility of noting who will finish first, and return to the mills at that time, to avoid confusion.
But if somebody shows up when somebody's clock reads 54 minutes, and not before that time, I'd tell them to patiently wait 6 minutes for their turn, and at a later time, interview the person who was running to make sure that they do, in fact, understand the guidelines.