Watch some of Anthony Diluglio's tutorials on snatches and cleans (where it's easy to 'bang' the forearm if not using correct form) at www.artofstrength.com . It's important to punch up into the kettlebell rather than let it flip over onto your forearm.
While you're learning the moves, try forearm guards (they sell them for tennis). You may have to double them if they are not thick enough. You can also make your own guards by cutting off the foot of a sock and using the ankle portion of it (probably two layers).
I still wear arm guards when doing cleans and other moves where the bell rests on my forearm, because even without 'banging,' the pressure of the bell itself can bruise my boney forearms!