It wasn't clear from your post if you wanted the feet to stay planted in one spot throughout the animation, but if that's what you want, you can follow these steps:

After turning on IK and setting the foot keyframes at constant, open the graph pallette and manually delete all keyframes (at least those associated with movement; i.e., you don't need to delete MT-related keyframes) in the legs IK chain after the frame where you locked the legs (usually frame 1, but I often lock at frame 31 --where I start my animations-- when using dynamic clothing).  That is, delete keyframes for toes, feet, shins, thighs, buttocks.  Leave the hip alone. 

And save your work before you delete groups of keyframes...