As noted--sprue cutters, a sharp knife blade, like a new No. 11 X-Acto, sandpaper. All may be applied, depending on the nature of the casting and the pouring block.
I have some 1/72 cargo castings--oil drums and jerry cans--with the pouring block on the bottom of the piece. I cleaned those up by sanding on a sheet of sandpaper laid on a piece of glass plate.
Thanks, Blind Pew, for your tip about warming the piece first with warm water. I never thought of that, I will try that with some CMK 1/72 figures I have to clean up.
Prost!
Brad