You contact the some way, usualy by calling the call centre and tell them that you won't (didn't, although it is better to be polite and do it in advance) be using your (unrefundable) reservation with PNR XXXYYY, pax: FlyingFinn, could you please have the taxes'n'charges back. If you get some muffling about it being non-refundable, just ask for a supervisor and eventually it will magically become possible. Then after some time they will post a refund to the CC used to purchase the ticket. If you paid cash I suppose you could get it in cash form from a ticket desk, but well... cash is obsolete anyway.

I suspect that they will slap some sort of administrative fee there as well, most carriers do.

Theoretically yes. Although the problem is that this way you are basically admitting that you used throwaway ticketing and thus breached the contract of carriage - in which case they have the right to reprice your ticket as a one-way (which might book into a very high = expensive booking class) and you might end up owing them money instead. I don't know whether it would make any difference if you come up with some sort of "Accidentally missed my flight" story or not.