the G&G blowback is a simple idea. air is bled out of the cylinder every time it fires which fills a small chamber which pushed the bolt back. It returns the bolt by a spring. There are 2 options:
1. use some hot glue to seal the opening on the top of the gearbox. There is a small rubber seal between the gearbox and the blowback chamber. this is what you want to seal up.
2. remove the spring that returns the bolt. only issue is that the bolt will flap about a bit.
both option involve disassembling the AEG to get access to the bits you want to seal/remove.