I think the two main banks operating at Suvarnabhumi are Siam Commercial Bank (which is reportedly charging this fee) and Thai Military Bank = TMB. I don't specifically recall a K Bank at the airport.
K Bank = Kasikorn Bank = Thai Farmer's Bank. Their color is lime green. Their logos are a stylized K (vaguely like the brushstrokes in Cathay Pacific's logo) and the grainy top of a stalk of a rice in a circle. They are fairly easy to find.