Sounds like play to me. Maybe you could suggest that your dad spend some extra time playing with him in safer ways to burn off extra energy. Then maybe he won't be as interested in going after ankles. If all else fails (I know you'd probably prefer not to have to resort to this), a quick spray of water from a squirt gun/spray bottle whenever he goes after people's ankles may convince him that this type of play isn't so much fun after all.