dunno what action bar you're going to hide but here's how it can be done for default one
Code:
/script if SHOW_MULTI_ACTIONBAR_1 then SHOW_MULTI_ACTIONBAR_1=nil else SHOW_MULTI_ACTIONBAR_1 = true end
/script InterfaceOptions_UpdateMultiActionBars ()
SHOW_MULTI_ACTIONBAR_<1-4> is probably bottomleft, btmright, right, right2