Modify his DNA so that his body can only accept one type of food. Make it so that you are the only one who can sell that type of food, and you only accept one type of currency. Unbeknownst to Xur, even though you CAN sell that type of food, you have made a solemn vow not to. Watch gleefully as Xur runs himself ragged trying to get enough currency to buy the type of food he needs, only to come away disappointed when you don't carry the food he needs. Added benefit: As he gathers more and more currency, his money bag gets heavier and heavier, which has to drag around making him weaker and weaker. Thirty days later, Xur collapses from starvation. Execution completed, feed his bones to the birds, and kick his severed head around the Tower like the soccer ball.