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yep fees prevent this type of Ddos. Smaller MB blocks make this cost get more expensive thus preventing prolong spamming. Dash might have this problem but their use of masternodes helps them subsidize big blocks. That is why I believe if their is one coin that can do big block it is Dash.
Steem no... due to RCs which depend on your steempower(after the base limit) so the built in spam prevention system is kinda like a collateral based system. the more steempower you have more Rcs(resource credits) you have. Or in the future you can rent out RCs but even then spamming won't work that well.