There is no way to completely secure trees, period. Thinking you can is a false sense of security.
The best security is discretion, a high wide wood slatted fence you can't see through, a lockable gate, BIG trees and game cameras.
Not talking to people that you don't know well about your trees is probably the best anti-theft technique there is. Bragging (or even talking about) your hobby to anyone who will listen will alert them to what's going on in your backyard. I tell a very limited number of people in my neighborhood about my trees. My trees are also not visible from anywhere outside my backyard--a point almost as important as not running your mouth. Bonsai visible to passersby are going to get stolen by idiot teenagers, or other dimwits that want it for themselves or to sell for drug money.
I have a high fence and all of my trees weigh over 25 lbs. I also have a twitchy bull terrier who will bark loudly when someone gets into the yard or makes a shadow back there. Since my trees are large, and the fence is high, stealing them would require a lot of upper body strength and at least one accomplice. That kind of organization mostly eliminates idiot teenagers and opportunistic thieves.
I am also installing game cameras at various points around the yard to record thieves' faces, etc. Link them to a computer inside.
Even so, there are instances in the U.S. and elsewhere in which entire bonsai collections from some notable people have been stolen, or individual trees targeted and stolen in organized theft. Some thefts involved backing a truck up to someone's yard and loading it up in broad daylight.
Bottom line, if someone wants your trees, your trees will be taken, regardless of any chains, dogs, guns, cameras you have.
One way to recover your trees once they're stolen could be implanting micro ID chips in them (if they're worth the trouble). I know some have tried this. It can work if the thief attempts to sell the tree into the knowledgeable bonsai community and honest people recognize it. The chip (which is the same tech used on dogs and cats) can be read using an automatic electronic reader. The chip carries a unique ID registered to the owner...
Of course you could get lucky with publicity
As sweet-natured as this guy is, he's making numerous mistakes, IMO. He likes to brag which is probably one of the reasons why his trees were taken. He also vastly overvalues them and isn't afraid to sling about $75,000 values.
Chain link fences BAD
http://www.fox13news.com/news/local-news/thieves-steal-bonsai-trees-worth-7500