Comments can't be erased, but they can be invalidated. This means that the comment remains in the artwork comment list, but is crossed out. Here's how you do it:
In the Viewer, click the Invalidate comment icon:

The comment will be crossed out, with a reference to whom crossed it out:

If the comment needs to be validated again, click the Validate comment icon (the eye). The comment will be activated and the reference to whom invalidated it will be deleted.
Watch this short clip to learn how to comment, reject and approve in Cway:
Written by Fredrik Hultin.