if i understand you correctly (with i probly dont)
you just need two commands
tank setfuel 0
removeallweapons tank
that will render the tank imobile and defenceless.
if its for a mission, then do the following
NAME the aa gun 'tank'
make a trigger (no prefrence on size),
in the 'condition' field, delete 'this' and add
(player distance tank) < 4
and in the 'on activation' field put
tank setfuel 0; removeallweapons tank; player sidechat "The AA is down, moveing to extraction point"
or somthing creative
have fun
SILENTHUNTER