This is the "Remembering Siri" story from Dan Simmons' Hyperion cycle. It was previously asked about and answered here: Story set on ocean planet, featuring floating islands
You seem to be looking for the short story version, which, per ISFDB, was published in the December 1983 issue of Asimov's magazine, and also independently collected in Simmons' anthology Prayers to Broken Stones. Many will recognize it from its inclusion in Hyperion. Per Wikipedia:
Simmons used it as the seed for Hyperion (in which it appears nearly
Here's the relevant quote from the story about the protagonist's sabotage of the portal:
"So it is with great pleasure that I close this circuit, and welcome
you, the colony of Maui-Covenant, into the community of the Hegemony
The thin thread of the ceremonial com-laser pulses to the zenith.
There is a spattering of applause and the band begins playing. I
squint skyward just in time to see a new star being born. Part of me
knows to the microsecond what has just occurred.
For a few microseconds the farcaster had been functional. For a few
microseconds time and space had ceased to be obstacles. Then the
massive tidal pull of the artificial singularity triggered the
thermite charge I had placed on the outer containment sphere. That
tiny explosion had not been visible but a second later the expanding
Schwarzchild radius is eating its shell, swallowing thirty-six
thousand tons of fragile dodecahedron, and growing quickly to gobble
several thousand kilometers of space around it. And that is visible
-- magnificently visible -- as a miniature nova flares whitely in the clear blue sky.