How to Prune Zantedeschia 'Forever'?
Remove the flowers as they start to fade, pulling the entire stem out at the base

Zantedeschia 'Forever' · Zantedeschia 'Forever'
Plant tubers in loam-based compost in spring. Plants may be grown under glass all year, at a minimum of 10°C; alternatively use as bedding plants in summer once all risk of frost has passed. Keep well-watered in the growing season. Bring outdoor pots indoors, or lift and store tubers in a frost-free place, in autumn. See Zantedeschia cultivation
Remove the flowers as they start to fade, pulling the entire stem out at the base