Cutting wild onions will only weaken, not get rid of them, because they grow from underground bulbs and will resprout. The best way to get rid of them is to dig up the clumps carefully, making sure that all the baby bulbs on the sides come up as well. If you keep after them for a couple of seasons you will win the battle. If you are OK with using poison, you can spot treat them with an herbicide such as RoundUp. Put on a rubber glove and cover it with a cloth one. then dip your hand into mixed herbicide. Grab hold of the onion tops and as you run your hand over them, herbicide will be applied. The weedkiller will translocate down into the bulb and kill it. This is best done in early spring as they appear.