I use nutted axles instead of QR and the same for the seatpost as well for a touch more security. But then I have more problems with vandalism then theft in my area and I get to bring my bike inside the shop at work so it only gets to sit outside locked to something when I'm inside a store shopping on the way to and from.
On thing that could be good is triangle or pentagon shaped nuts that you couldn't work with a regular wrench. I've seen them used on security system housing and automatic card swipe gates and such. Difficult to get by design and the same goes for the special ratchet socket to tighten and loosen them. Wonder where you could get some in the right size threads for a bike axle?