It could be 50/50, or maybe something else like 80/20, I don't know.  Just something that hurts the block withholder enough to severely discourage it, while getting the steady payout of a pool.  I just threw 50/50 out there, but maybe something else is better.  I don't know the exact sweet spot that would discourage a block withholder.  I image a block withholder getting 80% would give up rather quickly and make it not sustainable.  

The other thing that would seal the deal is to block hash rentals as they might unknowingly perform an attack.  However, even under the 80/20 system, any dishonest hash rentals might get paid a low enough amount as to make hash rentals not profitable, so that problem might be solved automatically with an 80/20 payout.  

Just been thinking these things over, contemplating the pros and cons of pooled mining versus solo mining as they each have advantages.  There might be a sweet spot that has all of the advantages and little to no disadvantages.