One the peeves me off to no end is when folks prevent a zipper merge by placing their vehicle in both lanes to prevent others from going passed them to merge ahead. Their reasoning is that people should all wait in line and using a long merge lane is "cutting", when in fact the blithering arseholes are impeding efficient traffic flow because they know better than the law. It's people who genuinely think they are right about traffic rules and creating and unsafe environment as a result.
An observation: There's a specific road in my town, on which if I'm unlucky enough to have to drive on during rush hour, is constantly plugged up. There are a few on ramps, where people pass down and then zipper in. Fair enough. But if I had to guess, about a tenth of the people passing are not people merging in from off the highway, but those already on the highway who want to pass the traffic standing still. This is one of my peeves, and if I see it happening I will be that blithering arsehole who impedes traffic (actually, this isn't impeding traffic but improving the flow, as additional mergers are shown to impede traffic).
I disagree with your decision to take the law into your own hands, and in this case think you are both wrong. Call the city and tell them the scenario you see, have them set up a traffic stop if it's common. They'll be glad for the risk-free revenue anyway. We already have enough police on the roads without citizens taking matters into their own hands as well.
Whether you're wrong or right (in this case, I agree, assholes using the ramp),
it's not your job, and is illegal to enforce traffic,
especially with your vehicle. You don't know what that other person is doing; they could be in an emergency. It's highly unlikely, but you don't know. If they're not putting you or others in danger, you, just let it be. It's a bad move all around, and in this case you are creating a more unsafe situation because you are inconvenienced.
This story likely isn't true (there are many iterations of it going around) but it is a good illustration of why it's good to just mind your own when in traffic, unless someone is in danger.
https://np.reddit.com/r/AdviceAnimals/comments/1kbhcn/i_gain_strength_from_their_tears_and_anger/cbnhvxv/