Unfortunately, with nukes you can only sterilize comparatively tiny
volumina, while globally distributing potential contaminants, the
updraft pumping dust right into the stratosphere. Andromeda strain
movie was bull, but at least they have gotten that much straight.
I really don't see any other way of sterilizing Earth by dumping
enough rocks upon it to melt the upper crust (giving plenty of
opportunity for particles to reach escape velocity, a la tectites),
or introduce a singularity within its volume which grows faster
than it evaporates.