Nettles form a mass of yellowish roots, from which they happily re-grow. Now nettles tend to be full of goodness (yes you can eat young ones) both for us and the allotment. Taking frequent cuts will, eventually, kill the plant off.
If you have a patch of nettles in a corner of your plot, use them as a compost mine. Take cuts before they go to seed.
Otherwise, dig out the roots and watch out for re-growth from the small pieces you are bound to miss.
Nettles prefer an acid soil and liming to a PH above 5.5 or 6.0 seems to really slow them down. Glyphosate herbicide is medium effective – may need 4 applications to kill off an established patch.
Where there are nettles, you will find docks. They have a long tap root from which they will re-grow. You have to dig out the root and then kill it. You can either leave the root to dry out or drown them in a barrel of water to do this.
Be careful about rotovating where there are docks – the root cuttings will all leap up multiplying the problem.
Glyphosate herbicide is effective, allow time for the roots to die before cultivating.