1. What is the best initial way to defend against werebeasts? From looking at the wiki, I assume it is to construct early defenses including a drawbridge to seal the fort if one appears. And have a safe zone inside the fort and alert all dwarves to go to the safe zone until the trouble passes? What works best?
The werebeast attacks only get announced when they are spotted by your dorfs, which is dissimilar from most other attacks on the fortress that get announced as soon as they're on a visible tile, so the seal-bridge method is typically a very messy process. It leads to situations where either your dorfs are out on the surface where they can spot the beast early near the edge of the map and they get sealed out, destined to die, or the werebeast isn't spotted till it's right next to the entrance and there's no time to shut it out.
You can alleviate some of this by either creating a sentry tower with a single military dorf stationed in it in broad view of the map and the entrance of your fortress ( If you're unsure of how to do military stuff, that maybe is another discussion/topic) or by chaining/tying dogs to various points on the map to act as sentries. using the (b)uild>(v)ropes/chains and then assigning with (q)
Hosting your general livestock outside is also a viable, if similarly messy, option. If your livestock are suitably fast then it will have to chase them down one by one, giving you plenty of time. If they all die then there's always the option to trade for more, and if you have a butchery/tanners workshop nearby then this turns into a mechanism of automatic butchery without having to do it manually.
The reason this works ties into your second question:
2. If I do that, will the werebeast simply eventually go away?
Yes. Werebeasts are only active for a few days of the month. In-game days aren't terribly long, though definitely long enough to allow an unchecked werebeast to wreck havoc. If you manage to successfully seal them off from your fortress, or distract them long enough, they will revert to their pre-were creature, usually naked, and they will flee the map.
3. Any other simple advice about werebeasts? Can I prevent werebeasts from appearing somehow?
Engaging with Werebeasts with military typically results in more werebeasts. Dealing with residents that are werebeasts is usually a very tedious, manual and delicate process to prevent further contamination/fun. The werebeast curse is only spread through bites, so carefully check the (r)eport logs of the beast's activities to see if it had bitten anyone during its uninvited stay. Traps are typically your best bet. I believe werebeasts are succeptable to cage traps, so if you line the inner-entrance of your fort with cage traps it will more than likely be able to catch them and keep them isolated. You can then put them on display, or seal them off, or do whatever you want.
I believe werebeasts are a form of savagery, as such low savagery usually means fewer werecreatures. I don't know if theres more complicated means of removing were creatures. There's probably something in the RAW/WORLDGEN files you can modify to stop them, but not other forms of attack.
