Using work from someone else's talk for oneself I work in physics. 

Throughout my graduate career, I heard about people who have been known to see work done in a talk and subsequently catch up to that work and then-some, and attempt to publish first. I ignored this lore assuming it wasn't really true. 

Now I am presenting at a conference where there will be someone (actually a big-wig) who has a reputation for this sort of maneuver, and I don't have a pre-print yet to post online. 

Any recommendations about how to mitigate the potential for this to happen (besides being ready sooner obviously, I'm working very hard on this work). Thanks.
Give the impression that the preprint is almost ready so that big-wig sees no chance to be first.