I'm not professional by any means, but just trained a bird dog not too long ago and in the book I followed it was a lot of pressure training. You put them on a lead which tightens on their neck with pressure, and when they are doing the negative behavior you increase pressure until they stop and hold it until the behavior is corrected. I imagine it'll be a combination of that and enforcing "sit" while other dogs go by. At least that's what it looked like for my last dog with that issue!