If your event is on someone else's relay, it's their event, even if you signed it. They possess it, they own it, they get to decide what happens to it.

GM, love the nim.

If you own your event (because it is stored exclusively on your own computer, or because you pay someone to remotely control deletion on their computer), then you can delete it by simply deleting. Same as any other file.

If you have given your event to someone else, on a computer that you cannot access or influence, then they get to decide, if it will be deleted. So you have to send them another event, politely requesting it be deleted. Because it's not on your computer. It's on their computer.

And, even if they "deleted" it, they might not even delete it. They might just mark it "deleted" and keep it, or put it in a different folder. Cry harder. Their event, their business.