I agree with this but I dunno if they'll implement it.

However, I have somewhat of a work around:
While you might not be able to block out sound effects from spells (without turning the sound effects off altogether), you can turn off spell effects for non-party based players. I'd highly recommend doing that and turning off display names on anyone who isn't in your party or on your friends list, or at least setting it to initials only. It makes things a lot easier to see and you won't have to deal with the Holy spam. (Although you still have to listen to it.)