This is what you want
Posted by: george4908 on Feb-06-13 10:10 PM (EST)
Lockable, steel reinforced tie down straps:
Not cheap, but if you have a couple of thousand dollars of kayak on your roof overnight somewhere, this is the best deterrent. (Hopefully your rack will itself lock to the roof.)
These come in two lengths. The shorter will handle most narrow sea kayaks. The longer will handle wider kayaks and SOTs.