The old joke , to shut up a guitar player put a piece of music in front of him, was based a little on fact that a lot of guitar players never learn to read. I found it helped me tremendously and I practice my reading every day.
Which begs for a new joke;
How do you shut up a sight reader? Take their sheet music and even their charts and then ask them to hang out and jam for a while. Don't tell them what the songs are or even the key it's in or what the progression is. Just ask them to play along with all of the other musicians.
EDIT: Don't ask them to take a solo. There's no need to TOTALLY humiliate them. LOL