For those of you who like the current system here is a quote that sums up exactly how I feel about this issue.

Quote Originally Posted by Rentahamster View Post
Having to activate the spell again in the middle of casting to cancel it is bad design. This also makes cancelling spells that are activated via macros really hard as well.
Now whether this is fixed by adding an assignable button to a controller/keyboard, or by moving/jumping I don't care it just needs to be fixed.