Hello, 

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The options are to remove it by hand or spray it with an herbicide. Hand removal is a good option for small stands. The plant is poisonous so be careful not to come into contact with it. Always wear gloves and a long-sleeved shirt when working with poison hemlock. Dig it up, put it in a trash bag and send it to the landfill. Be sure to remove the flowers before it sets seed. Herbicides are an option as well. 2,4-D, Dicamba, Glyphosate, and MCCC-P are registered for home use. When using pesticides always read and follow all label directions. If it is a large stand that needs to be sprayed, consider hiring a pest control company. Whichever method you use, there may be a seed bank in the soil so you will need to monitor the area over the next few years and remove any poison hemlock that reappears. For more on identifying and removing poison hemlock, refer to the following: https://solvepestproblems.oregonstate.edu/weeds/poison-hemlock

I have tried round up and crossbow. Neither herbicide has worked. Is there a specific brand you recommend?

It's not unusual to need to apply herbicide multiple times for a plant like this. The earlier in the development of the plant that herbicide is applied, the better it will work. Once the plants have gone to flower, any herbicide is less effective. In addition, each plant creates lots of seeds, so the potential for new plants to come up for multiple years exists from the seeds. When you say the herbicides didn't work, do you mean that they didn't kill the plants you saw above ground, or was there dieback and regrowth? If the second, it was likely a matter of timing relative to the plants' development. This page has a few more tips : 
https://solvepestproblems.oregonstate.edu/weeds/poison-hemlock
I would suggest multiple applications of herbicide as allowed per the label, and spot treating new plants as they come up over the summer- again, if you get them early, the herbicides are much more effective. Once you get some knockdown of existing plants, applying mulch can help block out light needed for the seeds to germinate. I hope you find this helpful, Nicole