You will need a bitmap editing program like Gimp or Photoshop to remove the things you don’t want in the scanned image.
Here is a long tutorial on one way to approach the problem. Say your ruler lines on the paper are a certain color. You can select just that color and it will remove everything of that color. Since the writing is most likely one color, it will leave the writing behind.
I use GIMP. It is not easy to get started in, but it is very powerful.
I sometimes just do manual erasing to get a clean image to engrave. You can do that in Paint, but that can be tedious.