There is no invalidating your nsec as far as I know. Deleting just removes it from whatever client you're using. You or someone could still recover it and use it.
Discussion
so one cannot delete a compromised account? once compromised that's it?? i think the devs here can do a better job than that.
That's one of the trade offs of decentralization. Devs could come up with a way to delete an account, but if all relays and clients don't implement it then it wouldn't be very effective.