I have had a few girls that have done all the drills perfectly only to get to the actual BHS and do the pike that you are talking about. If they haven't done the drills above, those may fix the problem, but there are still one or two that I have done all the drills with who would jump right to a pike. If the gymnast is small to medium, then you can try spotting this way. Spot the lower back with your strong arm. With your other arm, grab her ankles with your palm up and a finger or two in between. As she jumps, you hold her legs back from piking. If she is too large to successfully spot this way, then try spotting the back as normal while the other arm catches the legs as they come up too soon. You need to do this on a soft mat as the BHS will not be successful, the gymnast will fall to her back after you stop the legs. After she does that a few times, she probably won't like it, and she'll begin to pay attention to what it feels like. That's probably clear as mud! I hope the drills work for you.
Also as someone mentioned, have them reach for a line. We do this on an appropriately sized wedge and have them reach past the seam.