I just took some screenshots from the Dota 2 Alpha Tools of the trigger boxes of the creep spawns.
This will show you how far you need to pull the creeps and where you can place your wards to block the creep camp.
Since the triggers are placed boxes and don't go infinite high you can move the creeps out of the trigger boxes with spells like Torrent.
The rocket from Rocket Flare seem to be "flying" but it is still too low, thus entering the trigger box and able to block the spawn.
The same applies the other way around. Those boxes won't go infinite deep.
Making the river not an available place to block creep spawns.
The red marked part is where 2 of those trigger boxes cross each other. Placing a ward in this area will block both creep spawns.
Some of you might already know it, some don't.
I hope this makes some things clearer since guides only show you spots you can use, but won't show you all the possibilities you have.