A few weeks ago a fellow sup dropped in on a local prone surfer that I often see at this spot. The sup cooked the take off and caused the surfer to fall off. They entangled, but luckily no one was hurt. I paddled over to the prone surfer and asked if he was ok and I made a joke about "Fecking paddleboarders, eh?". I then paddled over to the sup bro and had some calm, but strong, words.
Best thing to do is, as suggested by others above, is to try and be the positive force of change. Politely confront people who do stupid things and try to set a good example to others with your own behaviour. Be an ambassador.
To be frank, all of my best 'busy lineup' sessions are with prone surfers. I stay out of the way whilst competing for the best waves, but I do not snake people, or drop in on them. I often leave the initial waves of a set - it cleans the lineup and leaves me free to enjoy. I sit down and chill, smile, and chat, instead of constantly paddling around. I don't paddle for every trickle that comes past. I always make sure that nobody is paddling behind me when I am paddling out, or when I am about to be caught out and wear a bigger wave on my head. I find my own peaks away from the main crowd, when possible.
In my experience it's usually the intermediate sup surfers who probably didn't come from a surfing background that cause trouble in the lineup. They are the ones who will drop in on me and don't even realise it, because they don't bother to look back over their shoulder to the inside, or see me and think I am too far away to make the section and catch up to them. I'll let it go the first time. If it happens a second time, then we have words. Many are completely clueless about basic surf etiquette and lineup rules. Most apologise sheepishly after an explanation of how things work and we end up sharing hoots and smiles. Others are just douchebags. They usually end up paddling back in. I've never had to resort to anything approaching violence; I guess my size helps. I also can do a mean glare with raised eyebrows
