How can I prevent students from writing answers on an assignment, then claiming I didn't see their answer? What is a good way to prevent students from writing an answer after you hand back a graded assignment (exam/homework) and claiming that you did not see their answer? 

It is clear to me that this particular student even used a different pen for their answer, and I am 100% sure that the answer was not there. I would have seen it, and I even remember double checking to see whether the answer was there.

This is the second time this has happened to me in this semester (different students), and both times I am sure that they wrote the answer afterwards and are trying to gain more points.
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.