Would it be feasible to attack the bitcoin network by buying and deleting large amounts of bitcoin? Could a government, determined to stop bitcoins, just buy a lot of bitcoins and delete the wallet afterwards? Are the bitcoins gone then?
The bitcoins are still there, but without the private keys they would be unspendable and it would be highly unlikely that anybody would ever rediscover the private keys.

However, such an attack would not hurt the bitcoin network: The previous owners would already have been compensated through the sale, and as the total value of the network remains approximately the same, the network's value would then redistribute to fewer units of bitcoin, increasing the value of each remaining unit. The attack would only make the bitcoin users wealthier.

Even if all but a single bitcoin were lost, the network could carry on: It has been discussed many times (see money-supply, divisibility), that it would be simple to adapt the bitcoin protocol to divide into smaller subunits. Currently Satoshi are the smallest subunit and there are 10^8 Satoshi per Bitcoin, but if the protocol were adopted to allow bitcoin to be divided to 10^16 units, there would again be sufficient money supply to serve a global market to make payments in any amounts.