An additional thought: you could align this with Larry Niven’s Known Space cosmology by having the Aliens be a weapon used in the war against the thrints – the Aliens not being subject to telepathic control, either. You might kill off some planets accidentally with them, but with thrints, you can’t really take half measures.
Who wins between,
Mighty Mouse vs Underdog
Hitting them with 5 gigatonnes of antacid would neutralize them.
Yeah! That “molecular acid”-eats through steel, aluminum, and human flesh-a cupful in a gallon of water will clean anything!
:::Groan:::
(Might need less if you use that brand that says it absorbs 25 times it weight in excess Aliens.)
The problem is the foolproof way of wiping them out, we know that the Aliens take on characteristics of their hosts DNA (ie: Predaliens and the animalistic one from Alien[sup]3[/sup]) which could imply that if you’re not careful, something from the hosts DNA might cause them to be immune to your bug spray.
Once they become immune to their creators can of RAID, you’d have to resort to nuking the site from orbit (it’s the only way to be sure) or sending down a battalion of ground pounders to make sure that every last Egg, Facehugger, Chest Burster, Drone, Warrior and Queen were totally eradicated from every inch of the planet.
Of course you wouldn’t realise that the can of RAID didnt work until your colonists started going missing, and by that point it’s almost certainly too late.
The built in kill switch after a certain number of generations might work, but again who knows what the random DNA combination might do to it.
That’s really a big problem in the design of the Aliens, whilst making them take on physical characteristics of their hosts might help them better adapt to their environs it also adds in too much instability in their programming.
i know i know. after the Aliens got out of control, the Predators were created to keep the population in check. 
in AvP3 Infested Ripley, spurned and betrayed by the Humans, will travel to the ancient home of the Space Elephants and discover it overran with Aliens controlled by an Overmind. in the course of wrestling control of the Aliens and killing the Overmind, she discovers that the Aliens and Predators were actually engineered from the same species - the Space Crabs that the Elephants loved to snack on.
in AvP4 Queen Ripley will launch an attack on the Humans while the [del]Protoss[/del] Predator will join in to make a 3 way war just for the hell of it.
years after all this Starcraft 2 will finally be released.