Value Vampire - A person who when present, sucks all of the value out of a room.
Owen, Tom and Jordan are together having a great time and are all adding value to each others mood. James arrives on the scene and unbalances the harmony by not adding any value at all and in fact sucking some of the value out of the situation and everyones mood.
When James leave's Owen might say to Tom and Jordan: "I'm Glad James is gone, he is such a value vampire". The mood would then improve as value is replenished.