Last week I had to "Break" a stack where there were four switches and I needed to use one outside in an IDF.
Executing the show run command showed there were indeed 4 switches in the stack-
switch 1 provision ws-c3750x-48p
switch 2 provision ws-c3750x-48p
switch 3 provision ws-c3750x-48p
switch 4 provision ws-c3750x-48p
In order to remove switch 4 I had to run the following command (In configuration mode)-
no switch 4 provision ws-c3750x-48p
After that- Power down the stack, remove the necessary cables, and re-cable the stack. Upon rebooting the switches you should no longer see the switch which has been removed.