What is the name of the device that destroys portals in Portal?

The device that destroys active portals in Portal is the Emancipation Grill. These shimmering particle fields appear at the ends of many test chambers and at other strategic points throughout Aperture Science's facility.

When Chell passes through an Emancipation Grill, it deactivates any portals she has placed. The field can also disintegrate unauthorized objects carried through it, such as cubes, turrets, or other testing equipment. This prevents players from carrying useful objects into the next chamber and forces each puzzle to be solved within its intended boundaries.

The term “Fizzler” is also commonly used for an Emancipation Grill, especially in developer commentary and fan discussions. That nickname comes from the way portals and objects fizzle out when they encounter the field. “Emancipation Grid” is another name heard in the games, but “Emancipation Grill” is the standard canonical term used in the question. It is not the portal gun: the portal device creates portals, while the grill removes them as part of Aperture's test-control system.