There isn't a way to protect it from the Lightroom export. But, if you have Adobe Acrobat, you can. Export the PDF and open it in Acrobat. Then, click File > Properties and go to the Security tab and set your options.
If you don't have acrobat (or don't want to shell out the $$ for it), you may be able to search online to find a similar, less expensive tool.
If you do have the $$, I recommend purchasing Adobe Creative Suite CS4 Design Standard. For about $1''' you get Photoshop CS4, Acrobat, InDesign (for page layout) and Illustrator (for logos and the like). I know it seems a bit expensive, but even if you only need two of the programs (say Photoshop and Acrobat) it's already paid for itself.