DeleteUsers.cs

public async Task<bool> DeleteUserById(ObjectId id)
{
    var filter = Builders<User>.Filter.Eq("_id", id);
    var result = await _usersCollection.DeleteOneAsync(filter);
    return result.DeletedCount != 0;
}

public async Task<long> DeleteAllUsers()
{
    var filter = new BsonDocument();
    var result = await _usersCollection.DeleteManyAsync(filter);
    return result.DeletedCount;
}