Answering in the spirit of your former question Players playing weird characters breaking the immersion of my group :
If you're GM let those player play their silly dysfunctional races.
Let them play this races in silly dysfunctional combinations with classes.
However tell them upfront (or as soon and clearly as possible):
"This race has no limbs. Its natural attack is to spit slime (or whatever, I don't even care to read its description). You won't get a functional and mechanically desirable result. If your idea of fun in a RPG is playing something dysfunctional (even if for role-playing reasons), then here you go: A fully dysfunctional snail monk. You can pray all you like, but not bend your non-existant knee. And you will fight like every other snail unless you find an ability of the monk that really fits, e.g. fully mental feats."
Major part of RPGs is about consequences:
You go around killing. Guards will try to capture you for murder.
You are a psychopath from a peaceful elven tribe. The tribe will ban you.
You are a snail. You can't hit or kick.
If you're a player: Support the GM in the decision. Demand for the consequences. Tell him what IG consequence your character will draw, if the snail hinders your character. E.G. abandon the snail. Even a paladin might reason "It is more of an animal than anything else, so the rules of my order do not apply". After all if there is no consequence for the snail-player, why should there be consequences for you?
You will see, if those players will stick to the dysfunctional race, because they like the totally different role-playing experience. If so they might earn a certain respect for it. But I'd expect them to drop it and go for another munchkin idea of disruptive egocentric stuff that combines with power gaming.