Players cannot physically block another as far as I can tell. You can move through each other, and even if someone else is in your way, you can select through them with the default Q and E keys.
As far as I know, other players also cannot block your menu screens. This includes UI elements for quests, banking, enhancements, character sheet, etc.
Now, players might not be able to see through each other because of graphical effects, which can be a pain at times, but you can still walk through them, select through them, and attack through them.
That being said, the other player isn't preventing you from doing what you want, and you're not preventing them from doing what they want, you're just the one complaining about it and arbitrarily trying to enforce your code of conduct onto them.
Pro tip: What a player can do if they find themselves in this situation is reach up under their skirt, grab their dainty panties, unwad them, and then move on with their life.