On iOS, please follow these steps to force close the game:
  1. From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
  2. Swipe right or left to find the app that you want to close.
  3. Swipe up on the app's preview to close the app.
 
On Android:

Swipe up from the bottom, hold, then let go. Swipe up on the Bingo Pop app. 
To close all open apps,  swipe up from the bottom, hold, then let go. Swipe from left to right. On the left, tap Clear all.