I think you should have made a blocking bet on the river the same as, or slightly more than, your turn bet. If he has the flush, he'll come in over the top and you'll fold -- paying him off another $6. However, if he doesn't, he'll probably either call or fold, as ripping allin after someone who's been aggressive the whole hand is much harder than doing it when you get a scared check after a scare card hits. What's worse? Tossing another $6 into the pot or losing $36 (or $18 in this case)? You only have to be right/stop the allin from a worse hand 1/6 times for this to be correct.