I'm frustrated by player misconduct...

[Impulse]

Unfortunately, there's not much you can do about what happened. People like that aren't exclusive to this game by any means, it's just the nature of MMOs. When you bring thousands of people from different lifestyles/backgrounds/upbringings together like that, you're bound to find an ass here and there.
In the future though, it's better to trust your friends, LS mates, and players with a good reputation and/or no widely known history of screwing people over just for their own gain. Just because you grouped with them once and they seemed fine doesn't necessarily they aren't one of the evils.
Now, I'm not saying not to trust anyone. People like that are in the minority. Most players (especially past FFXI players) should know how fast reputation can spread.

Conjurers can use shields. (I don't know if that specific one can be used though.)

Mage types can only use Wood based, leather based and Lantern shields from what I remember.