There's ways to add security features on your PDF that will prevent people from printing, editing or extracting elements from the file. There's also watermarks that can be added. BUT there's also ways to bypass all this, so it's not 100% reliable. I could also easily imagine it must have taken you a lot of work to do this work, and your client could easily invest in finding someone to remove any security you may add to that brochure.
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 :)
Having a contract is nice but that's never a guarantee to get paid. By giving low resolution files to your client and requiring your payments, you keep the control and power over the situation. The client won't have choice to comply. And that's fair too.
Here is a link with instructions how to quickly change all your 1800+ pages into a low resolution PDF with images/JPG only.
Create a PDF from illustrator without being able to see the individual objects
One thing for sure: Never give a PDF proof with vector and/or high
resolution content that is unprotected and NEVER EVER GIVE YOUR
EDITABLE FILES! Proofs should be low resolutions PDF or JPG, not "print-ready" or editable CorelDraw files! You give those files after you got your money. In fact, the editable CorelDraw files should be sold separately at least 3x the price of the project! Normally, designers do not provide editable files unless there's a budget for this.
One last thing... Please, get paid, get down payments as you begin your projects; split the full amount of that project in many milestones and wait for your payments before sending proofs or doing any revisions. The way you're telling your issue, it sounds like you haven't received any money at all and did an enormous brochure for that client. I can see why you might be very anxious about all this, a 1800+ pages booklet requires a lot of work!
If the client likes your illustrations but doesn't want to pay for them, remove the illustrations. That's it.