Unfortunately, there's no really good way of establishing that two pseudonyms are not held by the same person, in a decentralized system. Doubly so when legal enforcement isn't available. Of course abuse is harder the more arbitrators you incorporate, but expense and difficulty are higher as well.
It can be partially mitigated by requiring multiple arbitrators (e.g. a 4/5 transaction with 3 hopefluly independent arbitrators).