Can you move the boardmans 100 to 200 feet from the hives. That will stop any robbing that may be happening, but your bees will soon find it.
Boardmans are 'especially' problematic when there is nothing else available for bees to forage. There are pros and cons to Open Feeding and much depends on timing.
I open feed 1-150 yards away from the yard 'twice' a year, FIRST in spring before the flows begin and AGAIN in Fall after receiving a few killing frosts. You'll pick up some ferrels and some YJ's but chances are real good your bees will eat up most of whatever you put out for them. And if your hives already are filled with honey, they'll just consume it, instead of storing it and capping, leaving the honey for winter.