i'm happy to find you sources, Rabbi Hayim Soloveitchik writes on it extensively. The Ban on Stam Yaynam is older, Rashi for example write in a letter to his son in law Rav Meir that there is no difference in prohibition between yayin nesech and yayin stam and that people have become lax ind it as the gentiles no longer libate the wine.
A source is something I can look up. A sefer or book I can look up, preferably with a page number or the like. In case you - or anyone else here is interested - there's a program run by BMG called Shivti for baalei batim to learn sugyos in depth. There are chaburos/shiurim all over (including Aventura and Miami Beach) and they each learn the sugya from a booklet that's prepared and sent out to everyone. This year they're going through all the maachalei akum sugyos, and they just started stam yaynum this week - you can download the booklet
here, though it's a bit of a pain since they make you sign up and 'check out' to get the free download. I believe next week the focus is on maga akum.
However, the ban as we know it, is from the tosefites. i.e. that gentiles cannot touch the wine rather than make the wine. Then theres a whole debate regarding who can crush the grapes.
Now we're getting somewhere. You should really be more specific. The issur of stam yaynum is in fact a mishna in avodah zara. The discussion of maga akum is obviously less straightforward. But to be clear, that doesn't make it an invention of the baalei hatosfos. There are several shitos as to how to understand the issue of stam yaynum, which lead to different opinions regarding maga akum. But each opinion holds that their shita was intended by the mishna and gemara. ETA: And all the poskim say maga akum is assur, including the Rambam.
