Yes you can turn that off.
  1. Go to the TeamRaiser, and choose to edit it.
  2. Go to 3. Select Event Options.
  3. Go down to Related Actions and choose to Edit Advanced Options.
  4. Go to b. Define Event Options
  5. Go to Option 15. Registration User Name/ Password Assignment
  6. Uncheck the box next to: Yes, administrators may enter the user name and password for a participant they are registering 
You will no longer be required to add a username or password, when registering a participant from the backend.