^
There's a few tactics i use in such dire circumstances :
1. I let them know that the tone they have isn't much useable when i mix it with everything ( esp when u spot them using metalzones * shudders * ). Getting them to use different amps is much easier than guitars.
2. Let them know if that's what they want to use u will not be respondsible of what the final product might sound like : this usually causes them to doubt their "own sound" that they're comfortable with.
3.U need to know what kind of bands they like , or what kind of sound they're going for and use that to enforce the fact that they won't be getting that kind of sound with the gear that they have.
I usually never have to sort to such tactics only because if i produce the bands , i usually take em out on dinner or at a pub to discuss what their goals are , what kind of sound they're going for and what kind of gear they have and will let them know if it's anything useable . If it's not , i usually get gear hire for them and give them the hint that their amps will not be needed in the studio.not having their amps at the studio separates them from their comfort zone mentality.
also , knowing the group dynamics and the roles they play in the band ( e.g alpha male ) plays a lot in knowing how to convince band members into taking ur way with things.It's good to know and identify early on the type of people u're dealing with and knowing how to deal with them.I suggest reading a book called " the art of music production by Richard James Burgess " and most general sociology books.