That is what they're doing already, buying discs at retail. So no, Disney can't stop them unless they stop selling physical discs (or stop including digital copies with their physical discs).

They could also do a deal with Redbox to sell bulk rental-version discs at a lower price, which would not include the digital download code included with the retail copies.

But they won't, because Disney is the 800 lb. gorilla that can get whatever it wants by beating its chest and charging...

...the lobbying and litigating costs to its colossumongous IP protection budget.

Well, right, they could contract with them to mutually agree to stop by incentivizing them as you suggest, but I was talking more from a unilateral standpoint.