Why a high voltage AC always present in SMPS AC to DC power adapters
The internal transformer used to step down (and isolate) the power voltage has capacitance between primary and secondary - this can be around 10pF to 1nF depending on construction and size and is enough to produce several to several hundred micro-amps of current.
It can be harmful to sensitive electronics - the open circuit AC voltage normally produced on an un-grounded wall-wart might be AC voltage divided by two so this means 60V AC for an 120V AC power supply.
Grounding is the only method I can think of but, remember that the amount of current that can be supplied is not harmful enough to cause you any damage providing you are sensible and don't push the wires into your eyeballs or connect through your body to ground via your torso.