I see no reason to turn off my IS when shooting sports using a monopod (with a tripod it would depend on the lens used).
The 300mm f/4L IS lens provides very viable sports images when the Image Stabilization is turned on.
However, if your lens has a mode in which panning is possible (such as the Canon Mode 2 on its IS equipped lenses) I would suggest using that mode to allow some panning when desired or needed...