Here’s a clever prank for you: Make the victim’s computer actually type the word “SPACE” every time they hit the space bar. They’ll be lost trying to figure out what on earth is going on, and it couldn’t be simpler.
Just create a new AutoHotkey script and drop in the following two lines of code—the first one is to hide the tray icon, and the second sets up the hotkey for replacing the text.
You’ll probably want to right-click on the script and compile it to an executable before putting it on their computer. I’m not providing a download for this! Just stick it somewhere on their PC, launch it, and watch the fun!