The easiest way to select all the frames you want is to click on the first frame and hold the  shift key down and scroll to the last frame and click on that one while still holding down the shift key.  Now all the frames are highlighted it is a simple matter of pressing the minus button to remove the keys.  Just did it with PP2014 and a 500 frame BVH file, so it should work for you. 

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13