The most effective way to discourage bypass in a marketplace is by providing value in the marketplace-backed transaction. However, sometimes, you may prefer to disallow the direct message feature entirely.
How to disable the direct message feature
To prevent users from contacting each other directly:
log in to your Admin panel
go to the "Users / User rights" page
uncheck the "Allow users to message each other freely" box
What changes when users can't contact each other freely?
If that feature is disabled, then:
the Contact button on user profiles is removed
the Contact button on listing pages is removed
users can discuss with each other only by starting a transaction