If there is no spillage around the fuel cap I wouldnt worry about it because I doubt he used a funnel so some would have spilled..Then you can buy a cap that uses a key to remove, that would be your next option if he is making threats. I would also consider calling police so you have it on record. Let me know if you have questions, I hope he starts acting more reasonable.
It could cause damage, but nothing too serious..Worse case fuel would probably need to be drained/filled along with oil which would be $2-300 at shop but I would just buy the locking fuel cap and double check for signs of something beng added. Let me know if you have questions, thanks.