Essentially, you want to run bitcoind while connected to the network to download all the blocks, then disconnect it from the network so it no longer downloads new data. There are several possible ways to do this, from least to most drastic:

• Use the -proxy option to specify a proxy that does not exist.
• Use a software firewall to prevent bitcoind from communicating with the network.
• Run bitcoind in a virtual machine and turn off its network access.
• Physically disconnect the machine from the network.