Naive in the extreme. Knee-jerk reaction without thinking of where it would lead.
First off, many of the agencies already have 'exclusive only' categories -- some at the artist level and and some at the media level. OK, that "solves the problem, as stated," doesn't it? The agency does not want to get into policing where you put each and every image. You are either exclusive, or you are not.
Let's suppose, they DID go the "if your media is on VB, it cannot be here" approach, and it actually worked. What do you suppose would be the result? We all sit by the campfire and sing Kimbayah? Not likely. One of the three following scenarios (or maybe all) would play out:
1) The remaining agencies would look at each other, see there is now "room at the bottom, price-wise" and take over that new slot, selling for less, in order to undercut the competition. We are back to where we are now.
2) The remaining CBA (Controlling Block of Agencies) would choose the next agency to kick off the island. Hey, it worked the first time, so let's get rid of one more competitor! Where does that leave us? Everyone on the "new kid kicked off the island" needs to go remove their portfolio (if the New Kid even allows it), and the CBA gets yet a bit stronger.
3) If this goes far enough, then one or more of the "kids kicked off the island" will complain to the government about collusion and monopoly practices. They would be right in fact, since such things are strictly forbidden by law -- with the intent of protecting you (the user) from monopoly practices. The government would break them up... and we are back where we started again...