How to Prune sand leek?
No pruning required, other than to remove old flowered stems and foliage

sand leek · Allium scorodoprasum
Also known as: giant garlic; Spanish garlic
Grow in full sun and fertile well-drained soil. It is best to grow in containers where garden soil is heavy clay and prone to saturation over winter. Easily spread by bulbils and has the potential to become a nuisance without careful cultivation. See allium cultivation
No pruning required, other than to remove old flowered stems and foliage