Role is not defined by class.
Specify what role you want in your LFM instead of specifying a class, the player of which may or may not fill the role you want filled.
Examples:
Bad LFM: Need cleric, fvs (when you need a healer)
Good LFM: Need healer (opens up classes to FvS, Cleric, and other classes that can heal)
Problem solved.
The first LFM can lead to trouble, the second clearly indicates that you want a healer. If you get a BC with the second LFM you can feel free to berate the player when they don't heal anyone. Otherwise you got what you asked for.
Don't get upset when you get what you ask for because, well, you got what you asked for.