Adding lots of data to the transaction logs is one easy DDoS. Do so many transactions that the logs get to be larger than fit on most systems. With multiple Gigs of data already required, if someone was evil they could up the volume and keep people out of the game by making sure that that there were over a terabyte of transaction logs. Most machines won't have that kind of storage and would "fall off" the network.

Patient0 mentioned before I got to post this, the other attack I know would work. Destroying tokens. But it is a bit more complex than he mentions, but you can actually generate ECDSA key's that will work for one transaction, and then never again. A one time spend token that then self destructs for the person you paid.

I haven't been able to build anything that would work for two transactions. Which would be the most useful since you'd have a delayed "poison coin" but I don't see any reason it isn't computationally possible.