Untested Template

run_zombie_check()
{
	zombies = getaiarray("axis");
	array_thread(zombies,::dist_check);
}

dist_check()
{
	max_dist_for_respawn = 2000;
	too_far_away = false;
	all_players = GetPlayers();
	while(1)
	{
		for(i = 0; i < all_players.size; i++)
		{
			zombie_distance = Distance(all_players[i].origin, self.origin);
			if( zombie_distance >= max_dist_for_respawn)
			{
				too_far_away = true;
			}
			else
			{
				too_far_away = false;
				break;
			}
		}
		if(too_far_away == true)
		{
			self dodamage((self.health * 100), self.origin);
			//level.zombie_total++; //May not work when the round_spawning function in zombiemode.gsc has finished it's while flow
			//Could try making a seperate spawn script to spawn a zombie before killing this one which would keep the wave alive.
		}
		wait(1);
	}
}

The way I went about this with zombie train is:

zombies = getaispeciesarray("axis");

	for (i = 0; i < zombies.size; i++)
	{
 

		zombies[i] dodamage( zombies[i].health + 666, zombies[i].origin );
		playsoundatposition( "nuked", zombies[i].origin );
	}

What this does is kill of any remaining zombies.  I used a trigger mult...  It will end the round and start the next one.

Hope this helps! 

{
trigger = getent ( "axisdamage", "targetname" );
zombies = getaispeciesarray("axis");
   trigger waittill ("trigger");
	for (i = 0; i < zombies.size; i++)
	{
 

		zombies[i] dodamage( zombies[i].health + 666, zombies[i].origin );
		playsoundatposition( "nuked", zombies[i].origin );
	}
}

Get the axis(zombie) array and do the function dodamage(); on them.

also your model might go over the actual limit (which is about 15k)

Update the tool to the latest version here:

http://ugx-mods.com/wiki/index.php?title=Maya_2012-2014_Xmodel_and_Xanim_Export_Tool