Disabling the iOS Cellular Data Plan popup that asks if you would like to sign up for a data plan can be disabled by turning on Airplane Mode. Just remember to turn your Wi-Fi and/or Bluetooth back on.

Story

My wife and I both have iPad Mini 3s. Since we are usually connected to Wi-Fi, we share a T-Mobile SIM that we keep in the iPad is traveling and likely to utilize the cellular data. I got fed up with iOS constantly asking about a cellular data plan, so I figured out that Airplane Mode stops this.