Have had the same question myself and just found this posting. I know that clicking on the red “x’ symbol to the left will clear all the key frames on that clip, but havent been able to figure out how to delete individual key frames. I couldn’t see the grey arrows mentioned in the above post – possibly a feature of FCP X? I’m using FCP6.
However, I just tried holding down control when clicking on the green dot and this reveals a drop down menu with an option on it saying ‘clear’ – and that does the trick!