Feet and hands are two of the hardest body parts to immobilize thanks to the multi-plane turning bail;it of wrists and ankles.
Without toe ties, the least mobility for a foot comes from having the subject face down and placing your own weight on the backs of their legs at some point lower then the knees. This will kill th ability to flex forward (mostly) by pinning the limb art extension. You'll still get a lot of side to side though. So as a poster noted above, one of your hands will be needed to acomplish the task. Hold firmly with one, tickle with the other.
Myriads