Short story about a human who defies aliens who tell him to return to his planet
"Upstart", a short short story (about three pages) by Steven Utley; first published in The Magazine of Fantasy and Science Fiction, February 1977, available at the Internet Archive.
The two are on a mission to explore past their solar system, and they meet and board an alien vessel. The story begins inside the vessel, where they are informed by slaves of the self-proclaimed "Masters of the Universe" or something along those lines that they are not permitted to travel through space, and must return to their planet and tell humanity to never venture out again.
The Sreen have been very arbitrary and high-handed from the start, snatching our vessel out of normal space, scooping it up and stuffing it into the maw of their own craft, establishing communication with us through their Intermediaries, then issuing their incredible edict. They do not appear to care that they have interfered with Humankind's grandest endeavor. Our vessel is Terra's first bona fide starship, in which the captain and I were to have accelerated through normal space to light-velocity, activated the tardyon-tachyon conversion system and popped back into normal space in the neighborhood of Alpha Centauri.
The captain scoffs at this, and tells the lieutenant that if they do not return, humanity will keep sending people to the stars, so it doesn't matter if they don't return.
"If we don't make it home from this," I say at length, "if they never hear from us back on Earth, never know what became of their starship—"
"They'll just keep tossing men and women at the stars until someone does come back. Sreen or no Sreen." The captain strikes the door again, with the edge of his fist this time. "Sreen!" A bellow which, curiously, does not echo in the vast antechamber. "*Sreen! SREEN!"
The captain then forces his way past the slaves and bursts into the large chamber of the Masters, shouting something bold.
"Oh my God," I whisper to the captain, "oh, oh my God."
They are titans, they are the true and indisputable masters of the universe, the lords of creation, and they are unhappy with us. They speak, and theirs is a voice that shatters mountains. "WHO ARE YOU?"
The captain's lips draw back over his teeth in a mirthless grin as he plants his fists on his hips, throws back his head, thrusts oiut his jaw. "Who wants to know?"
