Is it possible to uninstall or disable iTunes in OS X?

If you want to just disable it, run sudo chmod -x /Applications/iTunes.app
It won't work anymore.

If you want it back, just sudo chmod +x /Applications/iTunes.app

On more recent versions of macOS, you may need to disable SIP:

1. Power off your device
2. Power on your device while maintaining ⌘ and R pressed
3. Once you reach the recovery system, select Terminal from the Utilities menu
4. Type csrutil disable and then reboot

If you want to revert that back to the original protection, run the same steps but replace with csrutil enable at step 4.

Check the current status with csrutil status in any terminal window.

Of course, check out the Apple support article, Removing iTunes for Mac OS X . (note: no longer links to the expected article, appears to have been removed.)

UPDATE: Here's an article for removing iTunes in Lion.