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, right-click on the Position attribute and select Runtime Expression (before or after dynamics calculation).
4. Paste the script into the Expression Editor and click Create.