I think it will not be a 'denial-of-service' attack but a 'denial-of-decentralization' attack. You can modify an old block and skip all the signatures calculation if everyone agree to take the last block as valid in spite it has a broken chain of calculations. Somewhat similar to the ETH/ETC hard fork.

But there has to he some kind of centralized group that bless the new block. Also, a new client and miner software must be released. Perhaps a new version every time an illegal item is included, or perhaps the program can check the list automatically in a website. In both case, it will break the decentralization.