Some time ago I've subscribed for 3 months, only to snatch a life-changing job offer the next day. Being sure that I won't find time to play properly and that the only stuff I could do anyways is to farm gold/alts and finish some achievements, I decided to request a refund.
No problems at all, I've received the cash two weeks later, the account was still active for 4 days after the request has been approved (didn't use it though).
(funny thing, shortly after the account has been frozen I decided that I will actually play, since I grossly overestimated my lack of free time. By the time I received the cash for the refund I already dinged my 3rd alt to 100 )
Actually, I have never had any problems with Blizzard support - they've helped me with refunding digital deluxe editions which I bought just to have preorder bonuses before I get CE versions, refunding game time, helping recover lost loot before Postmaster, getting to know some obscure RaF details, etc, etc. Always treated like an extremely valued customer, even up to the point of gifting me free stuff from time to time (gametime or Race Change, the most recent example).
The other thing that they do absolutely right? You can CALL them, seems like basic stuff, right? Yeah, you can't call many of the support centers for online games, including Riot Games. Blizzard call support is absolutely marvellous, by the way.