I have a friend who registered a few domains and a couple of them are obvious trademark infringements (although he hasn't had any contact about it). He registered these .mobi's for 3 years via godaddy and now just wants to undo his registration. He doesn't want to sell them or give them to someone else - just "undo" the registration so they they go back into the pool of domains available for registration. He isn't looking for any type of refund. He asked me if this is possible and I don't know. Has anyone done this before? UPDATE: Finally got an answer from godaddy - you can just cancel the domain via the admin and it will be removed from your account and whois will be updated within 30 days.
I believe you have 5 days to undo registrations, but not all registrars do this. Contact them. Good luck.
I would not put any content on the site(s). That way they hopefully won't be found. If the company tries to register the domain name and see it is already registered, they might send him a letter to turn it over to them.