boot_lock_bits_set(lock_bits)
Set the bootloader lock bits.
In this context, a 'set bit' will be written to a zero value. Note also that only BLBxx bits can be programmed by this command.
boot_lock_bits_set (_BV (BLB11));
Like any lock bits, the Boot Loader Lock Bits, once set, cannot be cleared again except by a chip erase which will in turn also erase the boot loader itself.