As Adelman and Dave state, most freeflows will actually originate from the 1st stage. If so, having a shut-off to isolate the primary first stage will only continue the problem - as the AAS will then begin to freeflow.
I had a reg-freeze freeflow whilst diving at 36m in an ice-cold quarry. It emptied a 15l cylinder in under 3 minutes. This was the time it took me to get orientated, reassure my buddy and begin an ascent. The cylinder had completley drained when I got to 15m. The rate of gas loss was incredible. I tried to breath the free-flow, but it coated my front teeth with ice (ouch!) I finished the ascent using a pony cylinder. No dramas (apart from sensitive teeth afterwards).
Whilst you can buy in-line 2nd stage shut-offs, you rarely see them used. That illustrates their worth.
For recreational diving, a freeflow is far from a catastropic failure. You could still breath from the freeflow. You should have a buddy available for air-sharing. You won't be far from the surface, so a controlled ascent using either solution is entirely achieveable.
If freeflow is a particular, personal, concern for you - then you need to address 1st stage shut-down. This will mean one of 3 common solutions:
1) Double tanks, with isolating manifold. Selective shut-down of valves preserves access to all of your gas in the event of 1st stage failure. If the tank valve itself should fail (rare) then the isolator will preserve 50% of your remaining gas. Gas management/planning ensure that you would always have sufficient air to surface.
2) Single tank with H valve. Whilst not as functional as doubles, this solution provides you with the option to shut down a free-flowing 1st stage and still access gas via another 1st stage. It is not true redundancy of gas. Neither does it provide a solution for tank valve failure.
3) Redundant cylinder (i.e. pony). You could transfer to a redundant cylinder (pony/bail-out) and shut down your primary cylinder. The only benefit to shutting down the primary cylinder would be to reduce distraction from the bubbles - but you may get lucky and be able to access the gas again at a later stage. If the free-flow is caused by icing - shutting off the cylinder may allow a thaw and the regulator to be re-used at a shallower depth. However, a prudent diver will have planned their gas requirements to ensure that their redundant cylinder has sufficient gas to get them back to the surface.
If you are doing deeper diving, or actual/virtual overhead environments, then you will need appropriate redundancy. In most cases this will consist of double cylinders with an isolating manifold.