Reddit does not (though I could swear there was a 'block' link for a short while when they were testing other things as well), but you can use the Reddit Enhancement Suite to get most of the way there (not sure if it ignores private messages)
edit: Hah, "block user" exists for PMs as TheHeretic...
Use http://cdr.thebronasium.com/sub/search
The easiest way to find the sub ID is to go to the appropriate regional Steam store, navigate to the game you want, and hover over the 'Add to cart' button. The address will be something resembling 'javascript:addToCart(SUB_ID_HERE);'. Just search for...
Yes, it'll say "XCOM Enemy Unknown (RU) PrePurchase" as the first line in the description section. The locked version might be what Steam starts selling once the game is released, but the prepurchase will remain unlocked.
I do agree that it's slightly risky (and against the Steam TOS since...
Your friend would need to open the support ticket, you'd just provide him with the scan of the key and receipt. Steam won't convert a key to an inventory gift, so returning it to TRU is your best bet at this point. Bar that, I'm sure someone in the trade thread is willing to put up with the...
For those wondering how/why people are selling XCOM so cheap, it's ~$19 from the Steam store in Russia and the game has no region restrictions. Users there buy it from Steam and resell it for $28-30 to make a quick profit.
Go through the process outlined here: https://support.steampowered.com/kb_article.php?ref=1673-IDGK-4694
It might take a week or two because Steam support is slow, but you shouldn't have any trouble.
edit: Apparently I can't read and it looks like you already found that :P. Hopefully it helps...