Which is why it has to be fair.
[GM ONLY ALERT - Non-GM's stop here, or Santa gets it!]
Let's be frank - there's only two types of players who are truly fearless. Those that don't care what happens to their characters, and those that already know what happens to their characters. (Or the GM really needs to sell it better - Roleplaying starts behind the screen, folks!)
Identifying which you have is important, as you may have to deal with them in opposite ways.
The former is tough. A player who is at the table because his friends are playing, rather than a desire to play, etc. A good GM can often bring this player around by giving them some individual attention - figuring out what would kickstart their interest. This player may need to feel like a hero - chances are they are not a big contributor, give them a chance to feel a positive boost from the game by being the guy who saves the day for a change.
A novice GM will often try to punish this player for lack of attention - this rarely works, and is generally counterproductive. A good GM makes sure his game is fun for all of his players, and an uninvested player doesn't start becoming invested because the character they don't give a damn about got stomped. Give them a reason to give a damn.
Summers players, however, sound like the later. If you play out a scene where a bad guy psychologically and/or physically intimidates a character, and they refuse to accept the result because they're Big Damn Heroes (tm) who know they have the run of the place...well, then they need taken down a notch.
Novice GMs know that winning is fun. But always winning isn't a challenge.
Many GM's are like bad parents. They make threats and ultimatums, then never follow through. If the players figure out that they're always going to win...the game loses a huge element of the suspension of disbelief. Ever been on the edge of your seat, watching a movie, and then something totally implausible or inconsistent happens? The moment you think "Oh, that is bull crap" the movie has lost you...you're no longer in the story, you're just watching a movie, and probably not enjoying it nearly as much as whn you were totally caught up in it.
Obviously, a TPK is counter productive, but setting up a situation as a challenge and then allowing it to BE a challenge are imperative. If they square off with a guy and fail an Intimidate check, don't tell them how to react - but you can tell them how they feel. But if they KNOW the guy can't back it up, you can't sell it.
So beat them up. Execute a hostage. Let them wake up with a horse head in the bed next to them. Implant a cortex bomb. Frame them for murder and extort them. Blow up their car/house/dog/Contact to send a message. Make the guy you warned them was a scary badass BE a scary badass, and let he dice fall where they may!
They will thank you for it, from the edge of their seats...if you do it well.
Some of it is play style - Pink Mohawk games are more likely to have fearless heroes than Black Trenchcoats. I am obviously more toward the darker end of the spectrum. But any game suffers if they players are in God Mode and know it.
-Jn-
Ifriti Sophist