I don't do sketches either, ever, even after they book with me, unless it's something so generic and vague it makes no difference anyway. I'll write a description of the cake on the contrct but forget the sketching.
I have lots of undercutters around here, and one person who tries to block other people by getting into package deals with the venues around here, or discounts for certainvenues. It's aggravating...You might want to try to tell people that if someone offers them a lower price in writing for the same cake that they can bring that contract to you so that you can see if you can match the price. I'm not saying to match the price, but if you get a few of those bringing you contracts from the same person you can see if there's a pattern of one person undercutting you and how much they're offering to take off. At least you'll know the specifics of what you're dealing with.