How can I disable ARC for a single file in a project?

It is possible to disable ARC for individual files by adding the -fno-objc-arc compiler flag for those files.
You add compiler flags in Targets -> Build Phases -> Compile Sources. You have to double click on the right column of the row under Compiler Flags. You can also add it to multiple files by holding the cmd button to select the files and then pressing enter to bring up the flag edit box.