You can't stop them
Listen, I've been riding bicycles for the better part of 40 years and I can tell you that if they really want it, you'll never stop them. However, you can do quite a few things to make the theft less convenient for them. I never ride anything that looks like much. I always put a cable lock on it (cable locks are not easy to cut with bolt cutters - just saws). I like to park my bike right in plain sight of the front door of the building in front of God and everybody. Usually there's a convenient sign post out there. That way, whatever they're planning on doing, they'll have to be in plain sight doing it. Now that I've added a motor, I'm thinking about something to lock the clutch and/or the ignition. And that custom-made deal that somebody else posted about that locks the forks would be very good. I'll tinker . . .