How to Prune leek 'Below Zero'?
No pruning required

leek 'Below Zero' · Allium porrum 'Below Zero'
Sow seed, under cover in late winter or early spring or outside in late spring, about 1cm deep in moist but well-drained soil that is rich in organic matter, preferably in full sun though they will tolerate partial shade; thin seedlings to 15cm apart, or, to get the classic white stems, lift and re-plant seedlings from spring to early summer in holes 15cm deep at 15cm spacings, and harvest from late summer or early autumn to winter; for more advice, see leeks cultivation
No pruning required