One thing that works good also is to catch a keeper or big bass where they just left from, and make sure they see it. We did that a Green a couple years ago, some guys trolled around us and got in front of us, we just kept fishing, did not say a thing, but get the net as both of us caught a couple of nice fish behind them, they left soon. Or better yet lob a gob of sting bait into their boat.