No,you can't make them less tall, you needed to buy smaller varieties if you wanted shorter ones. Usual practice in Britain is to leave them in the ground till first frost, then lift, remove topgrowth, leave in a tray of sand in a greenhouse or somewhere, stem side down, to dry out for a week or so, then pot them into a mix of peat and sand or compost and sand, till ready for potting up again in early Spring to start into growth. Should be started off inside and put outside once all risk of frost is past next year.