How do I kill individual particles?

The only way to kill a certain particle Id is to give it a lifespan of zero. For example:

//RUN TIME EXPRESSION FOR REMOVING PARTICLES AFTER THEY REACH
//10 ON Y AXIS
//GET THE POSITION OF PARTICLES
vector $pos= particleShape1.position;
//CHECK TO SEE IF THEY HAVE PASSED A CERTAIN Y VALUE (10 in this example)
if ($pos.y>=10)
{
int $pi=particleId;
//PRINT IDs OF PARTICLES TO BE REMOVED
print ($pi+"\n");
particle -e -at lifespanPP -id $pi -fv 0 particle1;
}

To attach this script to an emitter:

1. Show the particalShape and open the Attribute Editor.
2. Change the life span from live forever to lifeSpanPP Only.
3. In the Per Particle (Array) Attributes section press on the Position attribute and select Runtime Expression (before or after dynamics calculation).
4. Paste the script into the Expression Editor and click Create.