I'd like to cancel an auto-renewing subscription for a particular user in my app; is there either a StoreKit API or a process with Apple support to provide a transaction ID and cancel the subscription associated with that transaction ID?
How to cancel an auto renewing subscription?
You're basically talking about a refund, and if the user has contacted you directly with that request, you should, in all cases, politely direct them to contact Apple via iTunes. Don't offer to refund yourself (could be a common scam), or get involved handling the details.
You, the developer, can't do that. The user can do it through their iTunes Account quite easily.