Honestly, it probably won't even take that--and what I've found is that "fluff" players who tool up to "crush competitive players" are usually disappointed and get even more frustrated as a result.
Just ask them. I don't think I know anyone who would refuse to modify their army (within reason) if you just asked them.
What you shouldn't do is accuse them--they're not doing anything wrong, and people don't respond well to accusations.
Don't say, "man, quit playing those power-gamy, win-at-all-cost armies! you're un-fun!"
Say, "Hey, I'm really not having fun playing against your army, do you think you could do me a favor and maybe try a different type of game? Perhaps we could play a fluffy scenario, or, you know, you could at least leave out x, y, or z unit that I have particular trouble dealing with."
I really think that, if you take the latter approach, you won't run into much opposition. Most opponents will be happy to help you have a good time if you let them know that you're not having fun and what they can do to help.