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
save
And voilà!
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