How do you turn off the iOS 11 feature of AUTOMATICALLY sending confetti?
You can’t turn it off universally for sending and receiving, but you can block it along with other motion on your device.

Open Settings app > General > Accessibility - turn on "Reduce Motion" and turn off "Auto-Play Message Effects"



If the receiver has a device running iOS 9 for example he/she won’t be able to see it and won’t receive any readable text that a special effect was used.