How about the low, steady growl of a Rottweiler, just audible enough to keep someone from using it.
I've never watermarked any audition audio before, but I think it's worth it to at least try something.
We used to run a 10K tone on our spots we would give to our clients that wanted to "keep a copy for themselves" on 1/4" reel.
The only reason we figured someone wanted a 1/4" copy was to take the spot to another radio station, without paying us for the production.
That tone was enough to drive other stations nuts.
These days, I don't know what you could do, other than...whatever you used to do before everyone had mp3s, and before everyone started "sampling".
Hell, I had a couple of promos I produced for my station in Houston, featured on an FMQB sampler.
A few months later, I got word from some folks I know in Dallas, that a commercial running on a station there, had a voice in it that sounded an awful lot like mine.
I found out that the production guy at the station actually stole my bit from the FMQB CD, used it as a setup bit in his spot, and then the client liked it so much, they started running it...to promote their other stores...
IN HOUSTON.
Caught the guy red-handed.
I called him up, told him I had an agent, AND a lawyer looking into it, and that the invoice would be arriving shortly.
The spots were changed.