WASHINGTON — The Obama administration has sought China’s help in recent days in blocking North Korea’s ability to launch cyberattacks, the first steps toward the “proportional response” President Obama vowed to make the North pay for the assault on Sony Pictures — and as part of a campaign to issue a broader warning against future hacking, according to senior administration officials. “What we are looking for is a blocking action, something that would cripple their efforts to carry out attacks,” one official said.