I cannot be certain that this will work, but talk with whomever has Admin level access to Canvas in your district. They will want to go to Account Settings for your institution. Then find some settings for Users - manage login details. There should be a feature option for Self Service User Merge that may need to be unchecked. This should remove the option for anyone to merge users without being an administrator.
Try that. The admin may have an option to simulate a user account to test. Or they may create an observer account and link it to some students, then see if the option to combine is still available.
Let me know if this does or does not work. If needed, we can try to get someone at the next level to look into it for you.