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.

Another method of preventing unwanted gate travel is to put a stargate in low orbit above a planet and use it only for Threading the Needle. A method sometimes employed by the Wraith