How do you stop a particle effect? (SKEmitterNode)

Use setParticleBirthRate:0 to stop emitting particles. This is the most realistic way to turn off an emitter.

If you want it to disappear immediately then use removeFromParent.

If using setParticleBirthRate remember the original value to turn it back on later. for instance.

@implementation GameScene
{    
    SKEmitterNode *rocketfire;
    float rocketfire_birthrate;
}

// ...
-(void)init {
    // ... 
    rocketfire_birthrate = rocketfire.particleBirthRate;
}

// turn on the emitter (thrust rocket) when the screen is touched
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    [rocketfire setParticleBirthRate:rocketfire_birthrate];

}

// turn off the emitter on touches ended    
-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    [rocketfire setParticleBirthRate:0];
}

When you use the particle-editor you can set the maximum number of particles to create. It's the field below "Particle Texture".The official description is:

"The maximum number of particles that the emitter creates over the emitter’s lifetime. After this number is reached, no more particles are created by the emitter. Enter 0 to remove particle limits."

Also see: Particle Emitter Editor Guide

Of course, you should remove the emitter-node from its parent after it created the maximum number of particles. This can be done by creating an action-sequence that waits for a few seconds and removes the emitter-node from its parent [SKAction removeFromParent].