I've got the same problem. I've noticed that Cannon has a nice lock for thier down riggers, but not Soctty.

So, I sent Scotty and e-mail suggesting that they come up with some security solution. This is the reply I got.

"Thanks for the suggestion. Here are some locking ideas from our office in the meantime...replace star knobs with allen head fasteners so that you need a key to undo them. You should also fill the allen socket with sealant so that you have to dig it out before you can get the key in. Another idea is to have a small stainless cable made with loops in the ends (or a bike lock) that you can padlock together through your downrigger and something fixed on your boat. If you have a swivel mount, you can replace at least two of the star knobs and the tilt bracket star knob with eyebolts so that you can thread the cable tightly through them and prevent turning. I think most people just take their downriggers off when not in use, so we don't really have any other brilliant locking ideas! The allen key system works well apparently."

Not all that helpful. The allen head bolt filled with sealant will slow someone down. It's what I use for my racing bike seat to keep it from being stolen. My theory is to deter the thief. Just make it difficlut enought so they move on to an easier target. But, as was said, if they really want it, no reasonable lock will stop them.