This is probably summit stupid but, been messing around with stop casting in pvp for a while and developed a bit of a trick. Whats needed, Shatter 5/5 mostly, and some balls to take a hit.
Right what you do is, cast a frostbolt or w/e at something within the frost nova range, and as it nears the end of the cast, wack esc, if the "stopcast" effect works, the frostbolt will still be launched, but! Before your frostbolt starts its journy towards your target, hit Frost Nova, most of the time both the FB and FN hit at the same time, and the FB is aided by the shatter effect, but I've come to realise that FN has never broke due to crit of the FB, and keeps the target frozen allowing for moving away/another cast.
As i said before, this is prolly summit stupid, but it's a nice little trick i found imo :P