removal of invasive plant

Is there anything else growing in the area where this plant is? If not, you can spray it or water on a systemic weedkiller which travels through the leaves into the roots and kills the plants. If the plants are very deep-rooted, you may have to repeat this treatment after a few weeks. This type of weed killer does not harm the soil so after the weeds have died off you can rake them up, dig and compost your patch and replant. You can't hurry this procedure if you want to be sure that all the weeds have gone and that new plants will be happy and grow well. A good tip is to use a red watering can for weedkillers only, and keep other colours for normal watering.

It would be helpful to know specifics... first of all which plant are you trying to kill? find out first what it is...you could post a picture of it. 100 sq. ft is not a very big area...could you not double dig?
If the plant is indigenous it will come back no matter what method you use to remove it. I think you should do some more research before you poison your soil with herbicide.