How can I prevent students from writing answers on an assignment, then claiming I didn't see their answer?
Use placeholders

You can use


a sign or an indication that you would never use in any other circumstances (red cross, three question marks, etc.),
a short sentence ("No Answer Given", "I'm sure you knew it!", "You should have tried!"),
a sign that occupy the whole space (as suggested by Elizabeth Henning in the first place): slashes, X, Z,...




Don't try to catch the students that came back to you (supposedly) cheating, you just learned a lesson, and won't be caught again.