It can be partially mitigated by requiring multiple arbitrators (e.g. a 4/5 transaction with 3 hopefluly independent arbitrators).
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.