Sometimes you just have to remind the players that their characters may just have keener minds than the plans they the players have put forth. I've used henchmen in the past to ask "Hey boss, you sure that's a good idea?" or I've been a little more overt but a tad sneaky "Oh so that's you plan is it? Bob, look at your character sheet can you tell me what your Paladin's got for INT and WIS "
What brings this on, well this amusing post here: http://dungeonsdonuts.tumblr.com/post/150887605484/parisianqueen-when-the-party-youre-dming-for