The 4x6 rectangle with "Do Not Write In This Area. Thank you!" would be sufficient enough. Maybe you could write a poem or some sort of instructions on the first page of the book that tells guests where to write the congratulations.
Was it a naked lady? A clown? Wedding receptions are supposed to be FUN! If it didn't derogate you, why are you so upset?
EDIT - I could tell you the hundred and two things that went wrong at my wedding last month but it is in the past. I wouldn't worry about it. You can be bitter about it...