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.

I scan my student's assignments. There is a big printer with scanning function in our department where you can just put in the papers and it scans all of them at once.

Moreover, I do warn them beforehand that I will scan the papers and check if one makes a complaint. This has the advantage that the students will not cheat (in this way) and saves me the time from finding the student's scan in a huge pdf file and comparing the two versions.

Of course, to make scanning practical, you should tell the students not to staple their assignments (and only staple them afterwards), so as to be able to feed an entire pile of papers into the xerox.

Consider using electronic submission for weekly assignments. I have had success with asking my students to submit scanned PDFs of their handwritten homework assignments, via my college's course-management website. (We use Moodle, but I assume that Blackboard & other equivalent software have similar functionality.) Assignments are then graded directly on the PDF and uploaded as "feedback files". Any change to the uploaded file is not allowed after the official due date, so it isn't possible for the student to add additional information after the graded assignment is returned. In some sense, this is similar to the method advocated by user82630 above, but with the tedious work of scanning offloaded to the students.

The students have not generally had a problem scanning & uploading their files. My institution has many "multi-function devices" scattered around campus that allow students to scan papers to e-mail (as PDFs) for free. Many students also just take pictures of their assignment papers with their phones and upload those (though such submissions are admittedly harder for me to read, so I try to discourage that.)

This method might not be appropriate for a test, though, since it does require that the students be able to access the web to submit their assignment.

Use a two‑step collection process. First, have students write their answers on a separate sheet that you keep in a sealed envelope or folder until grading is complete. Only after you have marked the work do you return the graded sheet together with the answer sheet. This prevents the answer from being added after the grade is assigned. If you must return the graded assignment before the answer sheet is collected, require a written statement that the answer was completed before the hand‑back. Have the student sign a brief note (e.g., “I completed this answer before receiving the graded assignment”) and keep it with the work. The signature creates a record that can be checked later. Adopt a digital submission system where students upload a single file containing both the graded work and the answer. The timestamp of the upload shows when the answer was submitted, and the file cannot be altered after you download it. If you use paper, number each page and keep the pages together in a binder; any later addition would be obvious. Finally, reinforce the academic‑integrity policy at the start of the term and remind students that any claim of a missing answer will be investigated, with possible consequences. Consistently documenting the collection and return process gives you evidence if a dispute arises.