Iris

An iris is a device designed to prevent unwanted incomming gate traffic developed by Stargate Command. The iris is a metallic barrier several millimeters from the event horison of an active stargate. Anything that goes through a stargate with a closed iris at the destination will fail to rematerialize and be destroyed. Radio waves can pass through a gate, allowing SG teams to send confirmation codes to get through.

Analogs

The Goa'uld occasionally use a system involving an energy shield to the same effect, as did the Ancients to prevent unwanted traffic into their City ships