Unfortunatly the time to pull Garlic Mustard has passed. As soon as it flowers it will form thousands of tiny seeds (they are in little green spikes on the top of the plant). They are in full seed now. If you pull the dang stuff now, you will just spread the seeds and do more harm than good.
Let's all try to remember for next year. Earth day is the perfect time to pull it. It is big enough to recognize but hasn't flowered yet. Luckily, it pulls easily.