The client is clearly in breach of contract, so in theory you could sue him. But you'd only be able to claim the actual damage you've suffered, and it's possible that the court won't put much value on the link (but who knows?).
If this was me, I would do what you have already done, that is, write a polite email asking him to restore the link. If that doesn't work, try a phone call. Again, keep it polite and friendly. After that, I guess I would give up.
Then again, if it was me, I wouldn't have stipulated the link in the first place. The site belongs to the client, and it's not the designer's business to use it to advertise their own services, however discreetly. But that's just my opinion.