Generally speaking: your participants will use you when they need you and cut you out when they can do so cheaply. This isn't an indictment of them: it's completely rational for them to do this. But that means it's up to you to add value to the transaction, until the perceived cost of defection is higher than the platform margin.

Airbnb did this beautifully: they set up insurance for all their hosts, and so were able to frame "stay on our platform" as a safety directive.

You can also go the opposite way, but that usually leads to user-hostile antipatterns. E.g., Freelancer tries to detect all the ways folks might send each other phone numbers on their messaging service. This is both annoying and easy to circumvent.

If your rate of defection is high enough, no amount of tactical tricks will save you. Incentive and ingenuity together are too much.

When you're small and still have structural degrees of freedom, target the incentive by adding value to your offering if you can. If the problem only appears when you're big, you may have the resources to play whack-a-mole.

If you're small, have high defection rates, and also can't economically add value to your offering - it's time to move into another line of business.