If it's something silly, my response is usually, "Why do you want to do that?" When the player explains their reasoning to me, we quickly discover that either they're being a goober or that they haven't really thought it out, and they usually change their mind.
"I don't want to spend money on a silver weapon to fight the werewolf--that's too expensive. Instead, I'm going to carry a pouch of silver pieces, and when we meet the werewolf, I'm going to hold a silver piece against its skin with one hand and pound it into its flesh with my warhammer."
"You don't have to get a silver weapon, you could just buy some silversheen. You want to walk up to a werewolf and kindly ask it to hold still while you hammer silver pieces into its flesh?"
"Yeah. All I have to do is hold a silver piece against it and then hit it with my warhammer."
"Okay... So I'll let you touch a silver piece to its skin as a standard action with a touch attack, but you're basically sticking your arm out--that's going to provoke attacks of opportunity, just like if you tried a combat maneuver without a feat."
"I don't care, I'm not spending my cash on a silver weapon or a disposable magic item for a single fight. I'm going to 'think outside the box' and solve this problem creatively."
"You want to smash a warhammer right on top of your hand?"
"Uh, just on the silver piece--"
"If you want to hit the silver piece without smashing into your own hand, you're going to make an attack roll against the silver piece's AC, not the werewolf's AC, but you're going to also have to hold it reasonably steady against it's hide--basically, you're going to be attacking a diminutive object on the werewolf, so it'd be at the werewolf's normal AC + 8 to hit the silver piece without smashing the hell out of your fingers. Furthermore, if you hit the werewolf's AC but not the AC + 8, you've hit the silver piece and your hand. You'll take full damage from the warhammer attack and mangle your hand, making it useless until you heal the damage."
"But then the werewolf would die because I just warhammered a silver piece into its skin!"
"Not quite. A werewolf's hide isn't a piece of lumber, you wouldn't drive in the silver piece like a nail. You'd hit it and squish the silver piece between the werewolf's hide and your warhammer. There's a chance that you could get it in to its flesh if you held the coin on-edge and struck it just right, so if you managed to do all that... I'd give you a 50% chance of considering your damage from the strike to be from a slashing silver weapon instead of a plain ol' warhammer."
"Hmm. Yeah. I guess I didn't really think that through, but you're right... Smashing silver pieces into a werewolf's hide wouldn't be very easy. Maybe I'll just get a dose of silversheen instead."