The short answer is "just like the real world, no, there is no true protection." But the long answer is "Just like the real world, the protection is what you make it. It's possible to steal, but it's also possible for you to build walls and to protect yourself."
In real life, the kind of griefing that involves walking off with furniture and destroying places is pretty hard. In many places, having (for example) someone steal your table or set fire to your house is so unlikely it's treated like a natural disaster. In MMO open world games, this level of griefing is exponentially easier, so I need to know that the devs have taken steps to mimic the RL difficulty. There's also the added problem of MMOs being a game. Sometimes people leave them for long periods, because they're leisure not life.
The important thing is, can I set something up so that it is not touchable by other players? Can other players destroy walls? Unlock doors? Are there impenetrable materials, or is everything destructible? Do you have to pay for impenetrable materials?
Pretty much nothing player-build is indestructible.
There is sort of a thing to make total destruction hard, but my gut feeling tells me it's insufficient, but we'll see how it plays out. Each time you commit a crime against a claim your crime debuff increases. The bigger your debuff, the more black bile you lose each time you commit a crime. Braziers, which serve as defensive towers, deal black bile damage. If you run out of black bile on claim you'll be KO-d there for 24h so the owner can execute you at his leisure. So theoretically if you commit lots of crimes on a claim you'll be unable to destroy anything more without getting KO-d by a brazier, but this all depends on how high you can actually raise your black bile.
Also, crime debuff (which can last up to 14 days if you max it out) prevents you from moving your homestead, and you can be summoned and killed with crime evidence at your homestead if you're offline. Also, evidence left by you can be used on your claim by whoever is breaking into it to decrease his crime debuff. He'll still leave evidence for you to re-retaliate if you have a friend or an alt to do it.
At the end of the day IMO it's all about having a bigger and better organized group, because these blood feuds will likely last until one side is wiped out. And peaceful players don't tend to make such groups