Anyone can be scammed, and it doesn't matter if you're smart or stupid because a lot of the times you get scammed from a firm that seems extremely trustworthy. Always be sure to double and triple check whether or not the work you're doing is for a reputable firm or client, and don't be afraid to ask for some money upfront because in most cases employers should have no problem giving you that 50% upfront if YOU are a reputable writer.
You could use payment processors that make all charges final where there is no chargeback option, but that would create an issue with freelancers scamming clients. I personally believe that there is no middle ground and like in every business venture you win some and you lose some. Take your losses with a grain of sand and keep moving forward.