Spying on a bitcoin user after knowing all of their addresses I'm new to Bitcoin and trying to figure out some question about it and blockchain technology.

My question is simple: Is it possible to spy on a person, if the attacker (criminal, government) somehow got knowledge of all or most of the victim's Bitcoin addresses?

Furthermore, is it possible to know how much money this person has?

Is there a measure or mechanism to avoid such a person-address linking and thereby a way to stay anonymous with one's payments online?
If you know all the addresses the person has, you automatically know their bitcoin balance. Generally, it's not easy to know all the addresses associated with a person because creating an address costs almost nothing. Proposals like BIP47 create them on the fly based on the wallets of the two parties, so yes, there is a way to stay anonymous at least on the blockchain.

The thing you have to worry much more about isn't the bitcoin aspect, it's actually everything around it. If you ever order something from NewEgg using bitcoin, that associates the addresses to you right away. If you have an account at Coinbase, that associates you to your address. It only takes one slip before a determined adversary can associate you and your bitcoins.