There are two situations where the genre can be broken. One situation is when the players are unaware of the what the genre is, and the other is when players decide they want to break away from the genre. For each, there is a different sollution.
1. When the players break away from a known genre:
I think the only way that I would be able to do this properly is with plentiful scattering of the word "stereotypical".. i.e, "Are you guys up to playing a stereotypical star wars universe game?", or "Ok, when creating your characters, lets try to make them interesting within the confines of a stereotypical fantasy campaign."
The more you use the word, or its synonymous the more it will subtly put a constraint on what the players come up with. You'll be less likely to get players thinking "outside the box" when it comes to backgrounds. On the GM side, you need to make sure that your puzzles and characters also fit within the stereotypes else it might throw the players off.
The other way is to be overt about it, but I think that fails your "restrictive sense" clause.
2. When the players break from an unkown genre
I happen to have some personal experience with this situation when I was itnroduced to DnD via the Dark Sun encounters campaign. I had little to zero knowledge of the dark sun universe. Here is how the problem, I believe was solved well.
At the beginning of each session, the GM would give a recap. First, the GM would give an overview of the world: "Last time, you traveled through the sands of Arathis, a world sucked dry from the mages who used the life magic of the planet.. etc." In other words, the GM would sprinkle relevant world genre tropes into the basic description of the scenery and past events. Then, the GM would also give an overview of the politics in our situation: "The elf destroyed the halfling archer, while the Thri-kreen ironically looked upon him as food, in the same way that the halflings looked at the rest prisoners."
These types of 'off topic' remarks, gave us a really good understanding of the Dark Sun world without requiring us to read any materials before the game started, or between sessions.
Another suggestion is to try to get your hands on the DnD encounters modules, and look at how they introduce each session. I think they do a really good job of explaining the genre and tropes to the group, without any expectation of prior knowledge.
To prevent the situations mentioned in your original question I would suggest the following.
1. The Stormtrooper with a face, tired
feet, and bills to pay Perhaps remind the player that the empire takes care of all of their needs, and they have been bred from birth for war. Or make some reference to the clone wars.
2. The Lawful Good Paladin advocating Pre-emptive
Strikes and Genocide An omen from their patron's symbol showing disapproval or if the players can handle it, you should remove their status as a paladin and make them go classless.
3. The Archaeologist 'tomb raider' selling
items on the black market to pay for
drugs and whores Shock from the black market supplier that such a respectable archaeologist would even consider contacting them, or if the players can handle it, have a them get blackmailed!
Thank you gomad for the suggestions for 2 and 3. You make a good point.