It's difficult to identify the specific plant as there is no scale (how long is it) and the texture/smell of the plant (these are all clues.) However, it looks like some type of grass. It likely is some type of weedy grass and mechanical methods (e.g. pulling) is an effective method. In terms of 'keeping it out' that may be more difficult as the seed can be spread via wind and birds. Layered and dense planting can make it hard for weeds to compete and mulch can help prevent seeds from germinating.