Geez, it seems like the people on this board are all armed and pissed off! Hasn't anyone here heard the saying "It's easier to catch flies with honey than with vinegar"?
Seriously, the best tactics when dealing with poachers are to be nice and non-threatening (e.g. "Any luck?", "What are ya after?" etc...) and then gently suggest that the area is closed ("Yeah? I seem to remember something about them closing this stream a while back for those fish. I could be wrong, though. You got a copy of the regs?"). This lets them know that you (while friendly) are on to them. Most poachers seem to move on at this point. Suggesting alternative legal waters/lands as a better option also gives an extra push ("I'm not sure this creek is open, but I do know that the Zipperlip is open. They say they might have a good run this year on the Zipperlip, so you might want to check that out if it's suchandsuch fish that you are after.")
If they persist or are unrepentant, give them a friendly tip that enforcement does happen...("I don't know...there have been an bunch of gamies up here in the last month.")
Let them know the penalties, too. Don't call it poaching, though - people get upset and confrontational at that word. Use "fishing/hunting out of season". ("They got one guy around here last year for fishing out of season...took his boat and fined him a bunch, too. Seems harsh, but ya gotta pay attention to the calendar, I guess.")
If they don't get the clue at this point, bid them adieu, note their description/license plate, and head for a phone to call WDFW. Their phone numbers are available at
http://wdfw.wa.gov/enf/enforce.htm . I generally have at least one on my speed-dial on my cell phone. Tell the agent to not have the officer act as if responding to a call, but rather as if simply on patrol. WDFW folks seem to understand personal safety issues in this regard and have always obliged me.
Just my $0.02!