Change desktop wallpaper to rival sports team, which is bound to annoy them.
My pastor from years back had a Coke classic theme on his computer. I goated his computer to display a Pepsi theme. He was not amused.
Another time, I changed his screen saver to display my picture, just so he would know. Then I left. Another friend saw what I had done, opened my picture in paint, edited it so it looked like I was sticking out my tongue, and set a screen saver password. Yes, the goater got goated.
Similarly, I changed the associate pastor's screen saver to display the picture of one of the young ladies who worked in the office (no password). The AP was single. She happened to be talking to him the first time it flicked on. She gave him a quizzical look, he looked, blushed and said it must have been me. Her response was, "So, you're saying I'm not a catch?"