There is no means to lock players out of drawing. It's worth mentioning that the default color for any drawing is the player's color -- the box next to their name at the bottom of the screen; while a player can change that color, it might help in identifying the offending player. As far as steps to prevent it, I've found that creating a "landing page" or "OOC room" is useful, for several reasons. First and foremost, if you move the playerpage ribbon to the landing page after each session, nobody will see whatever you're working on if they show up early. It also gives you a good means to collect all the cats together when you're ready to start. I've encouraged my own players to draw in the OOC room as much as they please ( and boy have they ). They can let out all of their desire to doodle and then not doodle elsewhere when it's not desired. Finally, if you do identify who's drawing on your maps, you can use the Split the Party feature to send just that player back to the OOC room as a "time out" without resorting to kicking them from the campaign, if you don't want to. They won't be able to continue drawing in your campaign pages while in time out, and you have access to a punishment that isn't quite so severe. As a drastic measure, if you still can't identify who the problem child is, you could move the playerpage ribbon to the OOC room to get everyone in time out. Much like how my parents trained me to wear my seatbelt by pulling the car over until I buckled up again whenever I'd unbuckle myself. It's not the greatest solution, in that it's punishing the group for the actions of the few, but when you don't know who the "few" are, there aren't many other choices.