Keyboard set to "cat mode"

Open a tiny terminal window somewhere on the screen and run cat in it.

Whenever you want to protect the system from your cat, change focus to that window.

Not many people know this but this feature was an important design goal for the cat program :). Unfortunately, really clever cats (like my evil beast) know what Ctrl-C is.

If your cat is clever enough to figure out Ctrl-C, Ctrl-D, Ctrl-\ or Ctrl-Z, run cat using this sh script wrapper (/usr/local/bin/toodamnsmartcat.sh):

#!/bin/sh

trap "" TSTP INT QUIT
stty raw -echo

while true; do
  cat -v
done

Lock your computer.

It's a feature you probably already know how to use and it's super effective.
One can easily activate it with Super-L on most linux systems including the common Debian variants.

Ctrl-Alt-L on Ubuntu.

Windows-L on Windows.

Control-Shift-Eject or Control-Shift-Power on OS X.

All input (except a special unique combination) is blocked and thus provides effective cat-blocking.

It's available on most computer systems (including Microsoft Windows).

Open vim

Even the most intelligent cat will not be able to exit vim.