Text somebody daily fun facts on some odd topic, e.g. pigeons. When they try to stop it, reply like an automated texting system.
You: „Did you know that in certain religious groups in India it is believed that when a person dies, their soul comes back as a pigeon?“
Victim: „What is this BS?“
You: „We hope you are enjoying your subscription to the daily fun fact site „Daily Smidgeon of Pigeon“. To unsubscribe, reply with Unsubscribe.”
You: “Oh dear. Do you really want to terminate your subscription to Daily Smidgeon of Pigeon?”
You: “Message not recognized.”
Continue for as long as you like.