http://en.wikipedia.org/wiki/File:Celts_in_Europe.png
All I want is those dark green/green places gone.
I'll try once again, this time with references as to where I'm pulling stuff, carefully selected for your review.
RMcD94 - To get them gone, you have to get rid of the lighter green that, to coin a phrase, gave birth to them too, do you not? I guess you could go ASB and toss a comet at them or something, but that wasn't the point, right?
Plague, war, etc. will only rid you of them if you stretch those disasters, as witnessed by the thoughts of other posters in the thread. Strengthen the Romans, bring in plague, etc. -- all covered previously.
That's for the 0-1000AD time stretch (I guess you might add Viking genocidal colonization as a possibility? Or Christian vs Pagan genocide? Still stretching those too, right?)
Before 0AD however, there might be an opportunity.
Let me first discuss 'what is a Celt' - something necessary for the following discussion. Besides, if you're committing genocide, it's good to know the background of those you are obliterating.
From:
http://en.wikipedia.org/wiki/Celts
Celts (pronounced /ˈkɛlts/ or /ˈsɛlts/, see names of the Celts) is a modern term used to describe any of the European peoples of antiquity who spoke a Celtic language.[1]
The historical Celts were a diverse group of tribal societies in Iron Age Europe. Proto-Celtic culture formed in the Early Iron Age (1200 BC-400 AD) in Central Europe (Hallstatt period, named for the site in present-day Austria). By the later Iron Age (La Tène period), this Celtic culture had expanded over a wide range of lands, whether by diffusion or migration: to the British Isles (Insular Celts), the Iberian Peninsula (Celtiberians), much of Central Europe, (Gauls) and following the Gallic invasion of the Balkans in 279 BC as far east as central Anatolia (Galatians).[2]
From
http://www.associatedcontent.com/article/993530/history_of_the_celts_of_continental.html?cat=37
It is important to note, however, that the term 'Celt' is not an ethnic one, but a linguistic one. The thing that bound the Celts was not race but that sharing of the same language, Celtic. Celtic, a descendent of an older language known as Indo-European (whose other children included Sanskrit and Latin), has not survived except in place names. Its children, however, still survive in the modern languages of Irish, Scottish, Manx, Welsh, Cornish, and Breton.
Please note - the actual 'factual' origins of the Celtic Culture (not the Celts, defined by language) is debatable and no firm consensus has been reached by the experts. Some say Iberia, some say Danube, some even say BOTH - IMHO, it makes more sense to spread the language (and maybe culture) from the Danube west.
The majority view, at the moment, seems to be for an Iberian explosion to the west. However, this makes less sense if you consider the Hallstadt Culture, which becomes the heartland & source of Celtic Culture from 400 BC onward, is in modern Germany.
The truth may be complex enough to never know for sure.
I will skip over the issue of Celtic appearance as unnecessary.
Now, specifically, your areas of the dark and darker green are referred to as 'insular' Celts.
From
http://www.thefreedictionary.com/Insular+Celtic
A branch of the Celtic languages comprising those spoken or having originated in the British Isles and divided into the Goidelic and Brittonic groups
From
http://wapedia.mobi/en/Celt (which shows a copy of your map)
By the early first millennium AD, following the expansion of the Roman Empire and the Great Migrations (Migration Period) of Germanic peoples, Celtic culture had become restricted to that of British Isles (Insular Celtic), and the Continental Celtic languages ceased to be widely used by the sixth century.
Insular Celtic culture diversified into that of the Gaels, the Welsh and the Bretons of the medieval and modern periods. A modern "Celtic identity" was constructed in the context of the Romanticist Celtic Revival, mostly in Great Britain and Ireland
So it is these, the Gaels, Welsh, and Bretons that you are targeting for Elimination. (I still wince at the though of no more Scottish!!)
Now, I've constructed the above to illustrate precisely what you wish to eliminate, and where it came from.
This creates a secondary option, if acceptable. The first being destruction of their language between 0 and 1200 AD - only possible if conquered and/or absorbed or removed from the face of the planet. None of these seem too likely at first consideration, as has been posted in other replies.
(Although I had a thought from a question elsewhere, when someone asked what would happen to Britain if the Romans never invaded -- Would the Anglo-Saxons keep pushing west, overruning all Celtic language with their own? Would it work here?)
The second option, however, is to interfere with their development in such a way as to bar any Celtic speaking population from making their way out there to your dark & darker green areas.
Now to do that, you'd have to do some research based off the surrounding cultures during time periods.
Look at all the PRETTY maps!
http://www.eupedia.com/europe/neolithic_europe_map.shtml
Now you might be able to play with those cultures, push one a little more strongly and the others push more westward, until you get to a point where the Hallstadt/Celtic language ends up in Iberia and not in the Briton Isles... but even that's a bit dodgy if the Iberian Celts had decent water going technology and just sailed north (as some have indicated to me, but I haven't found ANY indication of this)