SetCombatMode "BLUE" should work, but takes time before it kicks in. CfgVehicles has an intriguing variable MinFireTime. If this is the minimum period a unit will fire on its current target, (a reasonable guess?) the 20 seconds it's set to is a l-o-n-g time to be under fire. The next command doesn't seem to initiate until tht 20 seconds is up. A target might survive rocks for that period, but not much else.
DisableAI might be a bit severe as it can't be re-enabled and removing weapons/mags seems clumsy, but probably OK for a cutscene.
SetCaptive true is perhaps the neatest solution if it has an instant effect - wish I'd thought of that one, want to get home and try it now . . .