We can't fix the moral character of folks, but, well, lets see about the lock. Now first you should no, no lock is impenetrable. All of them will fall victim to an angle-grinder after a bit of time. But it takes a pretty hardened criminal to stand there with sparks flying and the sound of an angle grinder going, all nonchalant like "I'm not stealing this bike".
So, what we really need, is a lock that can't be snapped free with either a bolt-cutter nor leverage.
Now, most u-bolt locks have this trade-off. If the shackle is large, you can get the bike locked to anything, but, someone can insert a pipe and leverage it apart. If the shackle is small, this risk is reduced, but you may not be able to lock to a post.
There is a second trade-off. The steel. Mild steel is flexible and can be bent into that U-shape, will bend before breaking when someone inserts a pry-bar. But it can be easily cut with bolt-cutters. Hardened steel is tougher to cut with bolt-cutters, but is more brittle, it may snap when you pry at it.
Enter the chain I just acquired (above image). Its big, its heavy. The one I acquired (shown above) the links are 15mm mangalloy steel, word-hardened. It cannot be cut with bolt-cutters. The U-bolt shackle is far to small to get any leverage on, and is also of hardened alloy, impervious to bolt-cutters.
Now, the challenge with my solution, its not light. In fact, the chain weighs near as much as bike v2! Clocking in about 7kg / 14lbs. But its going to take a pretty hardened criminal to walk up with an angle grinder and have a got at it, sparks flying. They will need to either make 2 cuts through the lock (14mm hardended steel, will take about 1 minute each), or 2 cuts through the chain (15mm hardened steel, will take about ~70-80s each). During this time it will be loud and lots of sparks.
Here's a guy cutting the (slightly thinner) version of the chain. Seems like people would notice.