That's not how it works for physical trespassing. The onus is entirely on the trespasser not to go places he's not allowed. If a property owner tells you to stay off their property, and you subsequently visit that property, you're trespassing no matter how few barriers you encountered to that visit. Why
shouldn't a clear "don't touch my web site" be legally enforceable, just like a clear "don't enter my unfenced, totally open yard" is?
Note: I'm not at all sure it should be enforceable, and the idea makes me uncomfortable, but I can't come up with a good reason.
This is a bit different from the common problem where someone guesses a URL and gets hit with unauthorized access. There, they merely lacked explicit permission to access the system in question, and the weird legal argument is that building a URL by hand implies unauthorized access. Here, the access followed a letter explicitly telling him not to access their system.