Destroy the value of a currency
First off, a quick recap of the obvious for completeness sake:
The traditional 1.0 copycoins are perhaps the easiest to bring down. Their network is secured by the miners (in case the coin uses PoW) and in return the miners get paid in coins they mine. However, if the value of the coin was attacked, the miners wouldn't have any incentive to mine it. They would need to be subsidized by the people running the network. As I discussed before when talking about Quark, a repeated 51% attack coupled with double-spending various crypto-to-crypto exchanges would quickly ruin the coin's reputation and get it removed from all services, thus destroying the coin's value.
Now with that out of the way, lets talk about some new stuff.
End the "blockchain bloat"
The topic of blockchain bloat has been a heated topic in the Bitcoin community for a few years now. It started with SatoshiDice sending a lot of 1 satoshi transactions, and the topic got more interesting with the advent of Bitcoin 2.0 technologies. By "Bitcoin 2.0", I mean Crypto 2.0 platforms that use the Bitcoin blockchain as a store of their data. For example - Mastercoin and CounterParty. While those systems use the Bitcoin blockchain because they can rely on it being a universally stored immutable record, at the same time they are reliant on the network accepting and storing their extra data. This could be their potential downfall.
There are some people out there that could be called "Bitcoin purists". They create transaction blacklists to hamper the propagation of transactions from Bitcoin 2.0s and other blockchain bloaters. While those blacklists might be effective if there are a lot of nodes running this code (which is rather unlikely), a more insidious attack would be to convince the mining pools not to include data from the Bitcoin 2.0 platforms. As mining becomes more concentrated in big pools, and the mining profit margins become thinner and thinner, a few pools might welcome a subsidy from a purist or a 2.0 competitor.
Moreover, due to how Bitcoin works, this blacklisting can be quite easy to execute. Someone wishing the bloaters gone could check each block for those transactions. If there were no such offensive records in the given block, the attacker could just send the subsidy to the address from the coinbase transaction. This way one can reward censoring pools without even knowing who the parties involved are. You just have to let them know the rules of the game through an email or some public announcement.
To counteract this, the Bitcoin 2.0 would have to rely on the goodwill of the honest pools, increased transaction fees to give the pools an incentive to include their transactions or a direct subsidy from the 2.0 developers to the pools to let their transactions in.
All in all, this seems like an easy way to carve out your competition in the future if the competition between the Crypto 2.0 platforms will start to become more and more fierce.
Many countries use the banking system and the flow of money as tools in their political regime. There is a lot of pressure to keep some countries, such as Iran or North Korea, from being able to deal with the rest of the world. However, in a decentralized Crypto 2.0 system, either everything goes or the system goes. This may very soon create a situation where the unstoppable force of innovation from the crypto space will meet with the unmovable object of nationalistic policies. In the end, only one will be able to prevail.
This issue affects a system like Ripple and Stellar the most. In those 2.0s one can freely trade between any pair of currencies. This means that if say, we have USD issued by a bank from the States and IRR issued by a bank from Iran, one can trade the USD for IRR just as easy as one would trade USD for EURO.
As the systems don't distinguish or discriminate between any currency or issuer, this makes the monetary sanctions unenforceable in the system without completely shutting the network down.
If this issue would ever come up, this would probably be the biggest legal battle a crypto system would have to face. Can a company develop a software that is by its very inclusive nature allowing some people to break the law? Can such a system be allowed to exist and run? Perhaps a combined pressure from a hegemonic government and oligopolistic banks would be enough to drive the crypto scene underground. However, given another outcome, we would see that the emperor has no clothes and progress cannot be stopped...
There is more than one way to kill a crypto. You can either attack its value to grind it down to dust, pay the miners to censor it into oblivion, or challenge the status quo and see what remains after the battle.