It’s a tough one. This is an issue that confronts pretty much every gym owner in the country. Unfortunately if one member trains or gives advise to another member for free there is really not much you can do. The line is crossed when the member “trainer” starts charging money for his “advice”. Only then you have the power to either ban them from the gym, or start collecting rental fees from them so they can continue using your gym as a place to train their clients. In other words, your options are limited.
If they are former trainers, they pretty much know how to go around you and continue with their “services”. It’s not unusual for members to pay the membership fees for their trainers to come and train them there. I think if you see these members “trainers” coaching, advising or training more than one person that should be a sign that most likely they have a side business going on. You should be able to distinguish the difference between giving a simple advice on a an exercise from training someone for a whole hour. You can ask your trainers to keep their eyes open and take notes. The usual suspects always return to the “crime scene”. You will need all the evidence you can get in order to make your case against them and ban them form coming there.