I think it will be good if you include Terms Of Service link on your main page when user is already signed up, at least I can't find it here
The link is on the sign up page right underneath the sign up button so that people see them before they start to use the service. I know you have already sen them but for anyone else wondering if you don't want to logout to see them simply open the site in an incognito window.
Since you represent freebitco, I want to ask you, is it allowed to use a VPN?
This is a question I've answered many times here. It's perfectly OK to use a VPN for privacy reasons. It's not OK to use a VPN to aid multi-accounting.
And what happens if I accidently created more than one account? As I see it's against Terms Of Service but if I made a mistake and still want to play on freebitco but don't want to get my coins frozen, what should I do?
You should stop using the other accounts. It's pretty difficult to do accidentally because each account requires a unique email address. I've had people signup with a gmail and then type in the same start of address but on hotmail on the signup page thinking it's the login page for example. But they are going to notice that their balance is different pretty quickly and go back to the original account.
Is this case solvable?
If someone did accidentally signup another account and then they stopped using it as soon as they realised it isn't going to cause an issue. If there has been a long history of large scale abuse then it's unlikely to be resolvable. I do review every case on its merits before taking the drastic step of banning/freezing accounts.