Context Question: "Why are we so worried about artificial intelligence? Surely humans are always able to pull the plug?
Answer "People asked a computer, “Is there a God?” And the computer said, “There is now,” and fused the plug.
I read it the same way. The computer fused the plug with the receptacle somehow (although it's technically unlikely), because this would nicely contradict the assumption that "humans always will be able to pull the plug" in its literal form.
AE speakers may not know that in the UK, with 220V mains electricity, each (modern) plug has a fuse inside - leading to potential confusion on the "paraphrase" in the OP of "fused the plug" (Even though this is true today it was not true when the story was written - but the OP has no information on that
Thanks , Egmont, for the source. Here's a copy on-line. In that version, which is much clearer, God "fused the switch shut" so the machine coud not be turned off.