Wait until the stems turn brown. Then you'll be able to pull them off.

you could dead head them to stop them setting seed [if they can] or leave them as Spritz suggests.

Thats good advice by Spritz and Seaburngirl. By letting the stems die back naturally you are letting the bulbs build up energy - to give you more/better flowers next year. Ive got some too. Planted last year, they grew to about 2ft. This year 3ft, next year........ ?