So the solution is to build a lot of Workbenches all around in the air.

No You can block 100% the respawn of Deathquitos and everything else.
I had some Plains islands and as soon as i had covered the whole island with Workbenches and/or Campfires the repsawn stopped completly.

So there are 2 Possibilitys
1: The deathquitos fly in from further away. and i know they do that from islands that not far enough away. As a tumbrule the Distance should be a bit more as a arrow can fly.

2: You missed a spot, can happen to everyone, had it myself and found the spot this way
https://steamcommunity.com/sharedfiles/filedetails/?id=2619750724

The workbench area of effect is a sphere so flyers could spawn above it then dive bomb you.