phlox [sorry for correcting your spelling :)are perennial so dead head them. you can then do one of 2 things.
1) leave until the spring and then cut all the dead sems down to ground level. if its a big clump then that is a good time to lift and divide the clump and then replant 2 plants instead of one.
2) at the end of october/early november cut the plant down to about 2" and leave until new growth in spring.

unless they are the annual ones , usually about 12" tall or smaller. In which case they can be pulled out and composted or thrown away.