I thought the only way to prevent people from spamming you on Facebook was to unfriend them, but at some point they made it possible to block invites from people without unfriending them! I don't know when they added this feature, but it's about fucking time.
Privacy Settings → Block Lists (at the bottom) → Block Event Invites
And there was much rejoicing.
After you've blocked all of your promoter friends who spam the living shit out of you three times a week, it's safe to turn Event Invitation emails back on.