Ability cancel button/hotkey

[Tarmunora]

Currently it isn't clear how (if it's even possible) to manually cancel aiming an ability (such as wizar'd prismatic spray), which can be an issue if you no longer fulfill the prerequisites to using the ability (dropping your staff).
It would be nice to have a 'cancel spell' button, or a hotkey (<esc>?) to cancel the spell aiming so you can click on other things again in a timely manner