1) First, check if the echo-causer is using a laptop with external speakers. If so, have them disconnect the external speakers. If they must use external speakers, like in a board room, they need to use a mic set up designed for that situation as explained here.
2) Second, have the person turn down the volume of their speakers. This can especially be a problem if they are in a room with an echo to begin with.
3) Third, the best and fool-proof way to eliminate echo is for the echo-causer to use headphones. Doing this will ensure sound from their speakers is never picked up by their mic.
4) Fourth, to reduce the chance of you causing the echo, use a high-quality external web cam. We recommend web cams from Logitech.