1.miners can destroy bitcoin if no one mines bitcoins anymore, nobody can exchange them. Miners basically get a commission because they use their processing power to process transactions  between bitcoin wallets.
So, if no one is mining anymore, which is unlikely for now, you will actually destroy the currency. Even if it is still worth something, you won't be able to exchange it, so it is like destroying it.
2.if  Governments all over the world restrict the contact points with regular money and can make it very, very hard to exchange Bitcoins. Which would make it useless for transactions outside the network.
3.The third way is to send coins out of an address is to use the private key that created that address. No private key, no ability to send or verify those coins.
One could potentially “destroy” coins by sending them to an address that has no private key, or to simply send it to yourself and erase all copies of your own private key. This will result in those coins being lost in that address forever effectively destroying them.