Make a program for a slave that refuse the call of master if it is busy.
It's up to you whether you give respond to slave every time or not.
you can add a concept of semaphore here (just like in RTOS)
use a variable called flag.
for whatever time your slave is busy kept that flag bit high.
and for whatever time your slave is free kept that flag low.
and make a program that if the flag is low then respond to the call of master or otherwise block it.
hope you get it.