How can I enable the security bit of a teensy board?
The Teensy 3.2 board is based on MK20DX256. The processor core is an ARM and not AVR. The best way to learn about security on the MK20DX256 is to read the documentation. 


  Chapter 8 Security
  8.1 Introduction
  This device implements security based on the mode selected from the flash module. The following sections provide an overview of flash security and details the effects of security on non-flash modules.
  
  8.2 Flash Security
  The flash module provides security information to the MCU based on the state held by the FSEC[SEC] bits. The MCU, in turn, confirms the security request and limits access to flash resources. During reset, the flash module initializes the FSEC register using data read from the security byte of the flash configuration field.
  
  NOTE The security features apply only to external accesses: debug and
  EzPort. CPU accesses to the flash are not affected by the status
  of FSEC. In the unsecured state all flash commands are available to the programming interfaces (JTAG and EzPort), as well as user code execution of Flash Controller commands. When the flash is secured (FSEC[SEC] = 00, 01, or 11), programmer interfaces are only allowed to launch mass erase operations and have no access to memory locations.
  
  Further information regarding the flash security options and enabling/disabling flash security is available in the Flash Memory Module.


The next step is to search for an open-source project that uses the FSEC setting. And by chance the mbed does. Here is link to how it is done in the mbed CMSIS Cortex-M4 Core Device Startup File for the MK20DX256. There should be the same setting in the Teensy Arduino core. Locating that is left to the reader.

Cheers!