Grass needs sunlight to grow so if you keeping pulling the grass by hand and snipping it down so the phlox will shade it you can make progress keeping the grass out of your phlox. The only real way to rid the grass roots and all from the flower bed is to use a deep digging tool with narrow blade such as a screw driver and dig out the roots of all the grass blades. Shear back the phlox after it blooms to make it grow thick and help block out any grassy weeds. Keep growing, Marianne Binetti