Currently, there isn't a way for us to restrict the ability to copy the signature tool, as the signature is simply added in as an image.

There is a way to embed the annotation into the document, so it is no longer an "annotation" and the student could not copy that. Under the "export options" for the file, selecting "Flatten Annotations", which embed the annotations into the PDF during the export. The annotations are no longer editable by the Kami tool, which would prevent the students from directly copying the signature as an image//annotation, but as it makes a new copy of the file, it does not work for pre-existing shared files/grading.