I've skimmed the thread, but admit to not having read it thoroughly. Here is how I see it:
-relative with ASD believes they've found the perfect gift for OP (considering her slipped disc) and spends an awful lot of money on it.
-OP is not interested in alternative therapies (fair enough - sounds like my idea of hell too), but is now looking for advice on how to deal with the situation.
-a lot of people seem to think OP should do something against her wishes, which she shouldn't, because it's a thoughtful.
From my perspective - as an adult with ASD who can agonise over gift choices - I think the vouchers were given with the best intention. To him, finding something that might potentially help someone with a problem then have - even more so if it's something they would enjoy - is a win in terms of gift giving. He probably hasn't considered beyond 'this would be good as it will help her back' - he hasn't thought it would 'cure' OP, but that it might help, hence why it's the "perfect" gift. He probably hasn't thought about her personal thoughts on alternative therapies, just that this must be a perfect gift as it could help with pain.
Ultimately, OP doesn't have to do anything she doesn't want/like, regardless of whether she was given vouchers for free or otherwise - I cannot stand being touched by anybody I don't trust, so it would be a nightmare for me.
You say relative - how close of a relative? I appreciate straightforward and to the point communication, even if it seems hurtful - if I'd given vouchers for, say, a craft activity (something I would very much enjoy), and the person I'd given them to hated the very thought of it, I'd prefer them to say and give them back so I could enjoy them! Would I obsess over my thought process for a long time afterwards? Yes, because I have a lot of anxiety over 'getting things wrong', but if it meant a £300 voucher would actually get used, I'd want to know!
Also, please don't call the spa/treatment centre and 'stress that he isn't NT', or that he has ASD and isn't capable of rational thought (unless you actually think this) - disabled people don't need co-opting to make problems for NT people easier, we're people in our own right (not aimed at the OP, just the PP who suggested it).