Okay, apologies for the snake pic - but I was trying to convey the message that there is effectively no way of preventing a determined thief from stealing your C5.
Yes, use a mega-duper titanium-tipped Kryptonite chain lock if you like, but at the end of the day it's attached to the wheel, so when the wheel comes off (only 2 nuts!) the C5 can simply be driven off, pedalled off or even just carried off!
Now, a nice big King Cobra would stop that bad guy in his tracks!