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.
this is always a thorny issue. Unfortunately, some trainers work undercover to avoid any kind of profit sharing. You use the term “our trainers”, and from that I gather that you do not have trainers as independent contractors. Is that correct? Trainers often resort to that kind of clandestine operation when the gym does not give the option as an independent contractor.
Trainers who typically work as such are also ‘out there’ they have a web presence and are not invisible. If they are trainers with a business, I would approach them directly if what they are doing looks like training rather than being workout buddies.
What does your gym contract specify regarding onsite training by non-personnel?
If you don’t have anything in your membership contract, you might have a problem enforcing this. You could post a policy or a rule change, perhaps.
As Karin mentioned, trainers have to have some way to attract clients, so they are visible somewhere. If you use a scan-in system at your club, you can identify that person and search for them on social media and professional fitness sites. If they are listed as a traveling trainer for hire, then you can ask them to cease operations in your facility.
Keep in mind, though, that sometimes trainers need to work out. I’ve purchased a membership at a facility 5 miles away from where I work so that I can have some peace and quiet while I exercise. If I work out with a friend there, I’m really just working out with a friend, even though I might know more than they do about fitness.