I'll preface this by saying I'm autistic/ADHD.
So, what's the issue that you need help with here? I've read through your comments and I'm not quite clear.
You acknowledged that you can stop the flow of vouchers so you don't get any more, so presumably that's not an issue?
So is it that you have the vouchers and they are messaging you asking if you've used them, and whether you enjoyed them? And that you don't know what to say?
Apologies if I've misunderstood but I'm assuming that's the issue.
Every time they ask, can you not just defer the matter - "with the baby and little one, I've not had the chance to use the voucher yet - might be a while to be honest." And then just leave it at that - would that resolve the problem? Just keep putting them off, and keep stating that it will be a while yet.
You don't have to use the vouchers at all. And they don't need to know if you'd rather not confront the issue.
I agree in principle that being honest and direct is better but some ND folk can also struggle with something known as RSD (rejection sensitive dysphoria) which could make refusing a gift feel like a really big deal, a huge rejection.
We obviously don't know your relative, and how they would feel about a direct conversation. If you think they might be sensitive about a perceived rejection, then just don't use the vouchers and just tell them that you haven't got round to it yet. Rinse and repeat. If at a distant point in the future they ask, you can just say "oh goodness, yes, used them ages ago - thanks very much! I can't remember much about it now but it was great! Shame I don't really have time for any more sessions..."
And if it's waste that you're worried about then maybe a different mindset is needed. Your relative feels good because they've given you something, a local business has benefitted from vouchers being purchased - that's two big ticks. Even if you don't use the vouchers, both of those things are still true so it's not a complete "waste". There's no point spending time doing something you don't want to do - but there are still plenty of positives to the situation. Even more if you quietly donate the vouchers to a charity raffle!!!