OK, I am not a working pro... but I've hit this wall before a few times... and it's usually an anal-retentive smart-arse who asks the question (I avoid kid's shows like the plague, so that's eliminated that one!)
There are two 'outs' I have in queue... one I have used, one I am afraid to

because it's a little 'bold'. But hopefully, in either case, the deck will be ungaffed—or at least a deck which CAN physically be shuffled.
OK, out no.1, tried and tested... as suggested above, have a 'back up' effect ready, something which CAN use a shuffled deck. It must be an effect which is impromptu, and something which isn't part of your 'set' of routines. I use a glimpse of the bottom card when the speccy has finished shuffling, and write a prediction etc. Then just a simple force trick usually suffices.
So... I hand them the deck and say "sure... and well you're at it, shuffle it really well"
They do... so then "Hey, you're kind of good at this... fancy doing the trick then?"
Using 'gentle' mockery of the subject will drive them to submission. And will hopefully stop another spectator getting the same idea!
Method 2 is even more bold... and so far, it's only a theory... but it's perfectly feasible that it could work... so, kindly oblige... hand the pesky speccy the deck, and take out another deck and carry on with another effect.
If that were me—I'd be gobsmacked, and feel a bit of an idiot. Come back to them at the end of the effect you are doing and ask "Satisfied?"...
With mates/family, I find the 'let me shuffle the deck' thing to be quite annoying... but I usually request "hey, come on, you don't get this kind of luxury when you are watching Paul Daniels on telly do you?"
Or some other such comedy.
In fact, perhaps comedy is the key.
The other key is to keep in control of the deck by taking possession of the deck... keep it close. Keep your trick well paced.
Apart from that, Nick... where did you get that hammer from? Are they cheap?