I have a similar problem, but it's the ventilation from a bathroom, so I don't want to completely block it up.
The problem is, that the outside hole is easily 6inches square, and only covered with a slatted grid, not a louvred or otherwise valved covering. The vent is, naturally, south facing (uh, that is the cold direction here), so whenever the wind blows from the south, we get a cold blast of air dumped straight into the bathroom.
The kids never remember to shut that bathroom door, and it's the downstairs one, so is in use all day.
An additional problem is that there is a pipe (maybe an inch and a half diameter) running down the outside of the vent, from dodgy plumbing decisions involving a previous owner and the upstairs bathroom, so I can't easily whip the old cover off and replace it with a better one.
tldr: is there any way to fit a wind-block on an external vent, that can accommodate a vertical pipe through it, which will still allow air to flow out of the house??