To stop certain resources from appearing on the index page, you need to override the indexQuery on your Nova Resource.

/**
 * Build an "index" query for the given resource.
 *
 * @param  \Laravel\Nova\Http\Requests\NovaRequest  $request
 * @param  \Illuminate\Database\Eloquent\Builder  $query
 * @return \Illuminate\Database\Eloquent\Builder
 */
public static function indexQuery(NovaRequest $request, $query)
{
    return $query->where('user_id', $request->user()->id);
}

The above code would only show the resources that have the same user ID as the current authenticated user.

Besides where constraints, you can also use joins to load related tables and much more. Below is an example I provided to someone on StackOverflow:

public static function indexQuery(NovaRequest $request, $query)
{
    return $query->join('devices', 'devices.id',  '=','works.device_id')
        ->join('device_types', 'device_types.id', '=', 'device_type_id')
        ->join('brands', 'brands.id', '=', 'brand_id')
        ->select([
            'works.*',
            'devices.device_type_id as device_type_id',
            'devices_types.device_name as device_name',
            'devices_types.brand_id as brand_id',
            'brands.brand as brand'
        ]);
}