If someone testifies against you in court, there are a number of ways you can get back at them. Of course it should be noted that you should definitely not try and get back at them, unless you specifically want to deepen whatever legal trouble you're in. But anyway, the possibilities are almost truly infinite. You could, for instance, "swat" the person. Or you could dump a can of orange paint on the person's car. You could, I don't know, toss a watermelon through the person's front window.