I don't know of any canonical ways to kill either of them, but since Sandy can absorb sand to add mass, as much as I think Clays from Batman TAS is an amazing character, I think Sandy would probably be able to at least contain Clayface.
Hard to say, though, since Clays seems smarter. That would be a fun comic, for sure! Well, fun or confusing, like Batman vs. Spawn: Battle of the Out-Of-Control Capes!