Are you sure that they're only doing it to frustrate? Some of the one-train routes can be critical for connecting longer routes (the two from Seattle in particular) and in a two- or three-player game it's easy for those to get blocked off early. So reserving them by grabbing them with your first move is a legitimate play if you think you might need them later.
If these moves are chosen just to hypothetically block, they're bad moves. They don't know you're going for those routes, they're not getting many points for them, and they're losing moves by trying to anticipate your moves, which, once you've seen, you can work around.
I don't block for the sake of blocking often, usually because I'm at the limit of my cars just trying to complete my own routes, and playing purely to block tends not to win you many games. Sometimes I will take a route slightly out of its normal way to obstruct another player but I won't tend to grab random routes nowhere near the rest of my trains just to stop an opponent. But occasionally if my opponent hands one to me on a plate and I can afford it, I will, because why not? It's a part of the game and a perfectly legitimate tactic.
