- You may be looking for the Star Trek: Voyager episode: "The Omega Directive".
Omega lockout screen
The Omega Directive (denoted by Ω) is a top-secret Starfleet general order requiring the Captain of a starship to notify Starfleet Command immediately upon detection of an Omega molecule.
The directive authorizes the use of any and all means to destroy an Omega molecule. The Omega Directive was deemed necessary because of the extreme power and the threat to interstellar civilization posed by even a single molecule. The Omega Directive also authorizes a starship captain to violate the Prime Directive if necessary to erase knowledge of the existence of Omega. Knowledge of the Omega Molecule and the Omega Directive is restricted to Starfleet officers ranking Captain or higher, and then only on a need-to-know basis.
Background
In 2269, a team of 127 Federation scientists, led by Professor Ketteract, were working on a top secret experiment entitled "Project Omega" at a classified research center in the Lantaru sector. The project's goal was to create an inexhaustible energy source which would end all needs for all other sources of energy, hence the name Omega. The research showed a single molecule of omega contained the same power as a warp core, which led Ketteract to believe a small chain of Omega molecules could sustain a civilization indefinitely. Ketteract, along with various cosmologists, theorized that the omega molecule once existed in nature for an infinitesimal period of time at the exact moment of the big bang. Ketteract even postulated omega was the primal source of energy for the explosion that began our universe.
Research station destroyed by omega molecules
The scientists were able to synthesize a single Omega molecule, but only for a fraction of a second before it destabilized, destroying the research center and killing all 127 scientists. An unexpected secondary effect was the rupture of subspace within a radius of 3.2 light years, causing warp travel to become an impossibility within this area. Starfleet Command realized the unimaginable implications: a chain reaction involving a handful of omega molecules could devastate and/or destroy subspace throughout an entire quadrant. This would effectively end interstellar travel for spacefaring civilizations in the quadrant. If such an event happened to Federation space, all communication, trade, and even contact, between Federation worlds would be rendered impossible; the Federation would be completely destroyed.
Therefore Starfleet Command suppressed all knowledge of the experiment and fabricated the story of a natural phenomenon occurring in the Lantaru sector which made warp travel impossible within the 3.2 light year radius of subspace devastated by the omega explosion.
Omega molecules being destroyed
Starfleet then enacted the Omega Directive, naming it after the last letter in the Greek alphabet, chosen to signify the molecule as the ultimate threat not only to the Federation but to every spacefaring civilization in the entire galaxy. If a starship were to detect an omega molecule, the following procedure would occur: the ship's computer would cut the engines immediately and lock out all computer access, displaying the Greek letter omega on every bridge console. Only the captain would have the top secret access code to clear the lock. In the privacy of his or her ready room, with the doors locked, he or she would give the computer a series of very high level security clearance codes. The computer would brief the captain regarding the detection of omega molecules, and then give instructions to implement the Omega Directive immediately – disregarding all other priorities, including the Prime Directive. The captain, absolutely forbidden from discussing anything about what was happening with any member of his/her crew, would contact Starfleet Command and inform them of the detection. Starfleet Command would then send in a specialized team authorized to use whatever means necessary to destroy the molecules. (VOY: "The Omega Directive")
The only known execution of the Omega Directive occurred in the Delta Quadrant on stardate 51781.2 in 2374 by Captain Kathryn Janeway, commanding officer of the USS Voyager. On that stardate, Voyager's sensors encountered the shock wave from a nearby omega explosion, revealing the presence of one or more omega molecules in the vicinity. However, Voyager was totally out of contact with Starfleet when the detection occurred, with no contact possible in the foreseeable future. Unable to call for an omega team to deal with the problem, Janeway therefore adapted the directive to the situation: she briefed her senior staff on the directive and worked with them to destroy the molecules. (VOY: "The Omega Directive")