This guy isn't spamming transactions on the bitcoin network. He is changing transactions just enough to cause a spam effect but not enough to have them rejected by the other nodes. I don't think that will work on bitshares. My concern is regarding the fee for proposing transactions. The way I understand our multisig is that you can propose a multisig transaction and broadcast it. It is then up to the other key owners to sign that transaction so it can be included in a block. I was wondering if this is a possible attack vector that would allow someone to Spain without having to pay fees.