Well, mathematically speaking this is another example of a less than ideal Nash equilibrium: Once there is the theoretic possibility of abuse without repercussions even honest people with good intentions have little incentive not to abuse the system themselves (because they would be at an even greater disadvantage if everybody else is abusing and they are not)

One quick solution to discourage self-voting would apparently be to categorically void monetary rewards on that. But cultivating good behaviour in people in general is a whole other challenge.