How can I protect my job from a client publishing it elsewhere?

So the best thing to avoid your file being used for printing is simply to lower its quality and make it an unacceptable quality for printing.

You can export a PDF from your Corel Draw file and then use Adobe Acrobat Pro to change all the pages to image-only, and to a low screen quality (eg. 90ppi and less.) Even if your client has the idea of going to a print shop or using another designer to work on the files, everybody will tell him/her the same thing; your files are low resolutions 🙂