Sickle or scythe for me dependant on the size of the area. I've used a strimmer in the past but the stems get wrapped around the head far too easily and it's a pain to untie. A brushcutter is best on your strimmer but I wouldn't bother if its less than 25sqm, there's something very therapeutic about cutting it by hand.
Nettles make great fertiliser if you bag them and drop them in your water butt, and the young shoots can be picked and made into nettle soup. They are very popular with the nettle aphid (which only affects nettles), which inturn attracts the hoverfly, so they are not completely without use in a garden.