The key to the strategy is to relax the requirement that a flag commit to a completely valid block, instead requiring only a valid block header. Miners can commence mining a flag as soon as they obtain a valid block header and can choose to abandon or continue mining the flag if spam is discovered. The split of hashpower between the flag and mining for a valid block should reflect miners' estimates of the cost of negative outcomes. This approach neutralizes the harm done by the attack and incorporates the work done to produce the spam into the blockchain cumulative Proof of Work, without payment to the spammer.