I think there may be several points on RP that you are missing about trading. Role play is based off from the rolls we see in everyday life. So for examples we need merely look around us to see what others are doing and then imagine ourselves in thier shoes. With that in mind, then.....
1) Many traders, especially freelancers, would fight to the death for the load they are carrying. Why? Because that load represents thier entire worth. If you are going to lose everything, then fighting is the only option you have. So when you rp pirating them, and they say no, that doesn't mean it's bad rp. It could very well be good rp.
2) Calling the athorities is not bad rp. The first thing a person does when attacked is yell for help. As a pirate you have to accept the risk that there is a lawful in range and that the trader will call it in time.
3) Threatening the ones that are taking from you is also not bad rp. A good bluff is the epitome of rp. So pirates, you are going to get threatened with bounties, reports to the law, faction sanctions, and general mayhem.
4) When a trader doesn't answer your calls for halt, that doesn't mean the trader isn't rp'ing. It just means he/she isn't talking to YOU. It could very well be that the trader is calling reinforcements, reporting his futile run, or simply playing the part of a smuggler not willing to wait for you to get around to pirate him/her.
5) Many people, companies, and countries refuse to give in to piratic and terrorist demands. Sometimes its moral, practical, patriotic, or simply just a person defending what's theirs. In fact, most law abiding areas refuse to give anything to pirates or terrorists. Saying, "No, I won't pay you to hurt me and mine, and I won't help you hurt others, so just blast away." is not bad rp.
6) 90% of all pirates died a horrible death at a young age. Every pirate wants to succeed, few do. Most die trying. Most pirates, thieves, terrorists only succeed a few times in life. So if most traders get away or don't pay, that's fairly typical, not the other way around.
7) Pirates are just as afraid of dieing as traders. Killing the trader that called for help out of pique when help arrives isn't always good rp. Try running now and then, rather than suicidal charges.
8) Most people would not trade with outlaws who have, or are known to have, pirated themselves or thier friends. So when a smuggler says he/she won't deliver to said outlaws if pirated, that threat should be accounted for. Not shrugged off by saying that others will deliver, especially when those outlaws do the same to other smugglers. Outlaws, smugglers are your lifeblood, try treating them as such. That means no pirating a smuggler known to deliver to your bases, even if the load they are carrying now isn't for your group of outlaws. That's just common sense.
Rp works both ways, and can cover many different situations. As long as it's not against thier faction, Id, and server rules, try working with it. No trader has to roll over and be submissive just because you want to pirate. I know I fight when somebody attempts to steal my things and hurt me and my family. I'm betting many, if not most, of you would, too. Trader ships don't have guns just for show.