Recently I had a game where I went for a risky Hatch-First build except before the hatch was close from being completed the Protoss walled off my main's base with pylons exactly like the picture below.
Then he continued to create photon cannons behind this wall. How do I combat this? I tried getting roaches up and moving spine crawlers up, however my spine crawlers miserably failed as he broke through one of the pylons and moved them to the spaces not covered by my creep in my main. How do I prevent this? How can I stop this after it has begun? Killing the probe I am sure would be the priority. This, however, can be a difficult deal when he protects the probe by the cannons behind it.
As a side note, I understand that many tournament maps have a neutral supply depot to prevent just this. Here is the link to the article.