I had the same problem once about 8 years ago during a very heave snow storm. I had water coming in my daughter's bedroom ceiling.
Thinking it was a roof leak, I went up to the attic. Instead, I found a 2 foot diameter pile of snow that came thru the gable vent!. I cleaned up the snow, and never had it occur again. I believe it was just a fluke. Hopefully that was your case also
But to answer your question, maybe you could try stapling a screening material on the attic side of your vent to minimize snow entry. A cheap solution might be to use a filter for a hot air furnace.