That’s a tricky one.
Best thing would be to keep your bees strong and narrow the entrances as soon as possible. Don’t let robbing start – it’s a devil to stop. If you set your boxes ‘the cold way’ I think that helps too – robbing wasps are then facing straight into the middle of the hive and are less able to slip in unnoticed up the front wall.
Were the wasps actually working the flowers or is the bush covered in aphids? Wasps are voracious predators at this time of the year as they have carnivorous brood to rear which they feed with aphids – in return, the wasp larvae give them a sugary secretion. Yuk. They only become a pest later when the old queen goes into decline, the numbers of larvae decrease and the worker wasps lose their sense of purpose and are driven to seek alternative sources of sugar. Without the wasps you might have to resort to insecticides and we know all about the negative effects of those things.
Otherwise you could set up traps but don’t put them anywhere near your hives or you will just attract wasps to them.
If you found the wasps nest you might be able to destroy it if it is accessible.