They're common as dirt here too. If you're growing them in your garden, be warned that they're great at killing off other flowers you want there. You might just wake up to find your grass is half nettle.
They're used for soup here, but are really just weeds, never heard of using them to spank before. If you do use them as a spanking instrument, be aware that they don't have thorns to sting, their hairs have some acid (methanoic/formic) to prevent them being eaten. Hairs break!
*Make sure the hairs are growing/pointing toward the person doing the spanking. That way when they come into contact with the skin the tips will break against the skin and still deliver their toxin, rather than snap and not penetrate skin. Also think about the direction of hairs against skin if using them in a nappy/diaper or anything like that.
*Each leaf will only last 4-5 spanks max. After that you're just hitting someone with a leaf. About as effective as shouting in Morse code, unless you're trying to look like Basil Fawlty hitting his car with a branch.
*Have some Dock leaves on hand. If the Dom gets a sting, take a leaf and rub it against the skin. It'll sooth it greatly, and probably be painful for the sub to watch too! Dock leaves famously thrive close to nettles, so if you're picking your own get a few of them too.
*The leaves near the bottom (the bigger, darker ones) work best. Their hairs are less fragile.