Hi AJ,
Some people use these:
http://www.ebay.co.uk/itm/Stainless-Steel-Outboard-Lock-Secure-your-Out-Board-Quickly-Easily-All-Makes-/150749434098?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D261124881570%26ps%3D54
It hides a mounting nut from theives and makes life awkward for them.
You can't stop them but you can slow them down a bit.
Jim