I wish the runes were "bound" to the numpad, and the "cast" was bound to "enter". I think it would assist in the annoyance of casting, keep the mysticness of the casting, and still prevent any spamming.

Google for "Grimrock" + "autohotkey" (also in this forum i think) and you'll find a script that does just that. You might need to adjust that for your screen resolution but, after that, it works perfectly