The best you can hope for is to slow them down. Most cables can be cut with bolt cutters, but most thefts are spontaneous acts done by someone who saw an opportunity and took it. Most of these guys aren't traveling around with a full tool kit.
Cable locks can be cheaply made.
To determine the length, pass a line around the kayak and rack, then add about 1'. Many Marine Stores (West Marine, etc.) Carry the end hardware, and use of their swageing gear is free. The last one I made cost me about $12.
Extra straps aren't a deterent, since they can easily be cut, even with a pocket knife.
Kayak Motor Kit
Rescue / Throw Bags
Recreational Kayak Paddle
Free Standing Boat Racks
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