I had a walker deliberately block the tow path on one ride. I had slowed to walking pace and followed up past a lock because we had about 10-12 riders in the group. I was a little ahead and when the path widened where he could not stop me from passing I pulled up along side and said as politely as I could muster. “There are a lot more riders following behind me.” His response was. “This is a footpath. Note the word footpath!” I replied calmly. “No, it’s a towpath, you retard.” Seemed to do the trick.
Sometimes it’s just easier to get down to their level.
99% of the time there are no problems, but when there are, I’ve found that politeness will have no positive effect on the situation. You might as we’ll be an ass because no matter what you say, they will think you’re one anyway and I always aim to please.