How to kick a player when they say a certain word in a sentence?

You'll have to use the .Chatted event. http://wiki.roblox.com/index.php?title=API:Class/Player/Chatted

In a localscript create your everyday function.

function chat()

end
game.Players.LocalPlayer.Chatted:Connect(chat)

Now we need to be able to get the message:

function chat(msg) -- while we're using "msg", you can use anything.

end
game.Players.LocalPlayer.Chatted:Connect(chat)

With this newfound data we will use string manipulation to find the word, we have two options: string.match() and string.find() The first one returns the word if its found, the second returns where the word is in the string. We're going to use string.match.

function chat(msg) 
word = 'apple'
if string.match(msg,word) ~-nil then game.Players.LocalPlayer:Kick('You said '..word)
end
game.Players.LocalPlayer.Chatted:Connect(chat)

Wow that was easy, right? Sometimes the answer can be resolved with a little research, try that next time.

Someone already beat me to this, but this script is slightly better.

In ServerScriptService:

game.Players.PlayerAdded:Connect(function(plr)
    plr.Chatted:Connect(function(msg)
        msg = string.lower(msg) -- If player uses weird caps, it will still work.
        if msg == "apple" then
            plr:Kick("You said the word")
        end
    end)
end)

The other person's script didn't have a line this one had. This makes so the player can say "aPPLE" and it wouldn't count. But because of this line, you could say "APPLE" and the line converts the letters to lowercase.