Only if freezing expends less energy than killing. If it doesn't, the most energy efficient choice would be to scan humanity and then wipe them out before they use more energy.
I'm confused what you mean by scanning. If you mean "scan and preserve the information in a databank" then it's a (perhaps very weak, depending of how much information relevant to us is actually retained) form of freezing I've been referring to (not necessarily literal freezing). If you mean "scan and compute some statistics, then discard the information", it is killing.
A comment to http://singinst.org/blog/2010/10/27/presentation-by-joshua-foxcarl-shulman-at-ecap-2010-super-intelligence-does-not-imply-benevolence/: Given as in the naive reinforcement learning framework (and that can approximate some more complex notions of value) that the value is in the environment, you don't want to be too hasty with the environment lest you destroy a higher value you haven't yet discovered! So you especially wouldn't replace high complexity systems like humans with low entropy systems like computer chips, without first analyzing them.