Robbing is NOT caused by feeding (*) - but by the excitement which feeding causes, which alerts the attention of 'robber scouts'. So:
1. Feed last thing at night - just as the sun sets.
2. Only feed a quantity of syrup which can be consumed by the morning, by which time the excitement will have died down.
3. Reduce entrance size.
4. Fit mesh anti-robbing screens over the entrances.
(*) especially sugar syrup, as it has no smell. You can place a saucer of sugar syrup anywhere in the apiary - even during a dearth - and it will be ignored, until a bee happens upon it by chance. Only then will it become an attractive source. ( This is why beekeepers sometimes drip syrup down into a hive fitted with overhead inverted jar feeders - to initially 'give 'em a clue'. )
But if you try putting a saucer of honey out in an apiary, it will be engulfed by bees very quickly - the difference is due to the smell which honey gives off.
LJ