Just publish the nsec for everyone to see. There's no more effective way of repudiation.
Discussion
But what about my private messages
I don't know what private messaging you use but if it doesn't offer PFS you're screwed.
I use this https://github.com/nostr-protocol/nips/blob/master/04.md is it not private and secure?????? What does PFS do????? I
Mebbe it's one of these https://acronyms.thefreedictionary.com/PFS