Is it possible to restrict any bike to a lower cc e.g. 600 to 125?
You can restrict the power using a kit, but to change the cc’s would require a different engine.
No. You can’t reduce the capacity of a bike (unless you change the engine).
If you mean can you reduce the power, ie for a restricted license then you can (for the most part) do it.
Wouldn’t reccomend it myself, all that weight and no power, not good in my book.
When I did my das a few years ago, one of the blokes doing it with me planned to ride a Blackbird restricted to 33hp!! Can’t think of anything more pointless/dangerous to ride.
ah yeh werent thinking of the power to weight, ill just have to wait!! 
There used to be a firm in Ware called Joy Engineering who would fit sleeves and smaller pistons to 4 cylinder Hondas. They used to convert a lot of 400/4’s to 250cc when the learner limit was 250
I think they are now in Bishops Stortford
I seem to remember Russell Savoury doing something like that to make a 750 Fireblade many years ago too.
All very costly i would imagine, but it can be done.
Not necessarily a problem mate.
Last (before current one today) issue of MCN (yes I know), Micheal Neeves rode a restricted 33 BHP 07 GSX-R 600. He said it was great fun and still topped out at just over 100 MPH.
Suzuki are doing a deal at the moment where they supply the bike and a restricted 33 BHP ECU which can then be switched to full power when you’re 2 year wait is over on a restricted licence or when you are eligable to take the Direct Access test.
Its somewhere on here
http://www.suzuki-gb.co.uk/bikes/detail.php?ref=gsxr600&y=k7
cool!! thanks for that Afro 
Hope they keep the offer till im 17
wow, just love the gsx-r600 in black but I dont know if ill have 7k when im 17 lol 
ill start saving up now.