What is the best way to add a watermark to a OmniGraffle drawing?

I found out that drawings I shared with other people are copied along without my permission. I would like to add a watermark (or other mark) to the drawings before I share the information on the drawing.
There is some similar option available in Visio, so I assume it is also available in OmniGraffle.
Who can help me.
regards
Peter