Have a splash page, should probably have one anyway, just the start page where the players tab sits when they are not on a map. I usually put some piece of art on the map layer as a background. Anyway if a player acts like a kid and starts drawing in between turns they get a time out and send them back to the splash page since you can split the party now. Just drag their name from the bottom of the screen to the splash page at the top. Yes bringing them back and forth might slow things down a bit, but hopefully being put in the corner will embarrass them enough, or they will see how disruptive their actions are and they will stop. I only recommend this because these guys are your friends, if random pick ups do this kind of stuff just ask them to stop then boot them.