Like anything else I guess, you could do some kind of DDoS against specific servers or somehow block related internet traffic. I guess the most viable thing to do would be attack the most centralized portions of it. You could try to put malicious code in open source mining software. You could DDoS or find vulnerabilities in bitcoin exchanges and things like that, where people actually exchange cryptocurrency for usable money. But attacking the distributed system itself probably won't get anywhere. You could maybe maliciously create servers that join the bitcoin mining network and try to fudge things up that way? But I'm not really knowledgeable about the actual protocol itself. So basically the same stuff that can be done with any internet system.