I would stop and check out his gear. If he is sporting all brand new stuff I would ask if he was new to the sport.
If he said he was, I would not only tell him the river is closed but WHY it is closed. Toss him a Fishing Regs Book (I keep spares), write down the URL for this forum on a piece of paper. And field any questions he may ask.
I know that is not the proper thing to do. But some people do not have any mentors to learn from. They just have a hole in their life and are trying to fill it with a healthy activity. While there is no excuse for ignorance if the guy was new I don't think he would want to be a part of this sport if his first outing resulted in a hefty fine. People that are new to Sport Fishing need to learn the cause and effect of their actions.
Now... If his gear was beat, he looked seasoned, and was a obvious poacher I would take a picture of him fishing, get his plate numbers, description, and report him post haste.
When it comes to matter of policing, people should not get involved other than to observe and report. But I find most new fishermen not only are more than eager to talk to people about fishing, but are also willing to learn. With all the information they flood themselves with on how to fish, quite often they can over look the major details.
I seen three people buy WIN cards, gear, and licenses at Canadian tire a week or so ago in NE Calgary. The person at the customer service desk did not hand out one reg book to these new anglers. And from what I could overhear from those guys they had never fished in their life. I seen them in the parking lot, gave them a spare guide, and showed them how to read it. They had planned to fish near Princes's Island park in Calgary. So it was a good thing I was eavesdropping.
It was a win-win. I got to talk about fishing, I saved these guys from fines, and they learned about spawning and watershed closures, how to de-barb a hook and why.
I seen them fishing near Southland park the other day. Assh*#s are catching --and releasing-- more fish then me.
Poachers suck. But everyone can use a mentor to show them the ropes. If it wasn't for my step father teaching me when I was young, that guy on the river could have been me.