This issue with keystroke combinations gets raised quite often on this forum.
If you are publishing to a browser-based output what is most likely happening is that your web browser already considers Ctrl + C a reserved keystroke combination (for Copy command) and therefore expects you to be copying something with it. The browser 'sees' these commands before your content does.
So the browser is your issue, not Captivate, and there's really no way to prevent it from jumping in on the act.
I haven't published it yet. This was just from previewing (not in a browser).
I am going to work around using this keystroke combination. I was just surprised I could get ctrl+r to work, for example, but not ctrl+c.