What if a multi-billionaire organisation were to buy a large amount of bitcoin (for example 30%) only to destroy the private key associated to the wallet and brag about it all over the internet ?
First the multi-billion organisation will have to invest a lot of time to acquire 30% of current supply and destroying private keys would just be denying themselves access to their loot and this would also mean less supply more demand which translates into rising bitcoin value 
First, would it be possible to prove the loss of the private key ? no
that won't be relevant if their actions are true we can see it in the value of bitcoinsSecond, How the market would react ?
The only effect this would have on the market is a low supply of bitcoins has the BTC is withdrawn from circulation.