Been giving that some thought. It is my understanding that most board by throwing up some type of grappling hook and line
Even if the ship owner won't pay for it, the crews could equip themselves. Tools suggested.
Bolt cutters lashed to railings near locations that might need to be defended.
Some type of heavy object that can go down on the ascending pirates.
At first I was thinking of something as simple as a 4x4 post that would certainly maim anyone it hit. Better yet would be if the post had some device that would allow it to be slipped onto the rope or cable. At a fast descent this would certainly break fingers and hands, knock them in the water and keep them from any further climbing. Maybe someone else that knows of expendable stuff on a ship can think of something.
Certainly something like a Molotov cocktail hurled down on the small boat below would quickly rendered it useless.
I can imagine a small anvil being tossed over into a little boat might significantly knock it out of action.
My point, if the ship owner and insurance company forbid firearms, look for other means to be aggressive and fight back. A good trip to the hardware store before one sailed might save their life.
They would need guns with a fair amount of range. The way I understand it, the pirates have RPGs. They get within communication range and tell the boat let us board or we fire the RPG at your ship and sink it. That is the strategy that they would have to have an effective defense against.