Can Lepton`s automatic flat-field correction (FFC) be disabled?

Yes, there are three flat-field correction modes for Lepton. For cameras with a shutter, FLIR sets the default at the factory to FFC, but it is possible to change the mode to manual FFC. (The third mode is for commanding FFC without using the integral shutter.) The mode is selected using the SYS FFC Mode Control Command, described in Section 4.5.15 of the software IDD. In manual mode, the camera will not perform FFC until commanded.