We have always had a policy of no lending / borrowing gold or gear in the guild. The one and only time we ever had a problem was 2 years ago (almost exactly) when somebody decided to disregard that policy. When scammers know your guild disallows behaviours that enables them, they go elsewhere.

Maintaining certain standards for recruiting helps filter out those who are not into earning stuff for themselves also, but it is being careful who you invite to your home that is key. That and having officers who are willing and able to enforce rules as needed.