I keep getting text messages from a random number after I told them to stop?

They don't text me every day, but they've texted me at least 6 times and each time I told them "Stop texting me, you have the WRONG number." Finally I blocked them. They were using an iPhone because the messages were blue, hence iMessage.

Today they texted me again today saying "I'm waiting for you at the Cemetary." I texted them again and the message bubbles were suddenly green, meaning they more than likely have an Android now. I texted them "For the last time, stop texting me, I don't know you." And this time they replied back "But you do know me. I'm your worst nightmare."

I told them to F off and then blocked them--again. Legally, if they keep finding ways to contact me, what can I do? I swear I have no idea who this person is. I've never seen their number before they began texting me.

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