You can play with In-Game boosters disabled by playing "Classic Mode," accessible from the Start Games panel.

Tap "New Game" from the Home Screen

Scroll down, and you will see "Scrabble Classic."

Tap on it, and you will be presented with a list of players to start a match with, or you can search your opponent by name or user ID.

Games started through the Classic Mode will be played without In-Game boosters enabled.