Approach, speed up and fly past. If it's during a climb they'll more than likely know you're back there by the sound of your tires, pedaling or coughing. If it's on a climb they can be considered advanced recreational at the least and should know how to handle a bike, even if surprised by a silent passing cyclist. Sometimes while being passed on a climb it's nice to be acknowledged with a "Hey" or "Nice day for a ride," or at least a finger flick. If you're doing really well on a climb and someone still passes you without saying a word, you can pretty much reason they are in a pain cave the likes of which you can't understand. To them you're just a pebble to avoid. Salute them either silently or physically as they pass.