Hi Peter -- There's an undocumented but well-established **`"snowshoe"`** access tag. Just add **`snowshoe=no`** **`"snowshoe=no"`** to the trails in question.
If all foot traffic is prohibited, regardless of the footwear, just using **`"foot=no"`** is probably easier, though you could still add **`"snowshoe=no"`** to be extra explicit.
If foot traffic is prohibited regardless of the footwear but *only* during snow, you could try **`"foot=yes"`** + **`"foot:conditional=no @ (snow)"`**, but in that case I'd definitely also add **`"snowshoe=no"`** because you never know if someone's actually going to read the conditional access tags.
By the way, if there's a sign or a published regulation that corresponds to this restriction, it would be a good idea to cite that in your change comment.