No solution, I'm afraid. Anyone can fork bitcoin, or freicoin, or any other crypto currency at any time and try to get people to use the new spinoff coin over the old one. This isn't the first time that it has happened to bitcoin either, just the first time that it had any significant (albeit minority) support from miners, users, developers, etc. There is not some crypto magic that would save us from this outcome -- I can think of generic approaches to replay prevention that would work no matter the underlying crypto system (e.g. XOR the signature with fixed data).