If you frequently hit Ctrl-S by accident, and don't consider this a feature, you can turn off XON/XOFF flow control using
$ stty -ixon -ixoff
then Ctrl-S/Ctrl-Q will be normal characters without side-effects in your terminal, you'll probably just see (depending on shell, terminal, ...) ^S on your screen when you hit it.
To turn it on again, use