sandbox.bashrc defines the following functions: sandboxon() { export SANDBOX_ON="1" ; } sandboxoff() { export SANDBOX_OFF="0" ; } libsandbox checks only for SANDBOX_ON, so sandboxoff() should set this variable (and not SANDBOX_OFF).
added, thanks http://git.overlays.gentoo.org/gitweb/?p=proj/sandbox.git;a=commitdiff;h=94d3b1443ebf3536ab92335caad85cb4b414136b