8bet is able to detect multiple accounts, perhaps it will be better if they block any account associated to the suspected/proven account that tried to abuse their bonus. Other better solution is to change how the free spins code works, posting it here every day without any requirement will always invite abusers to try taking advantage from the free spins code. Although there is a wagering requirement for the winning amount from the free spins, I do believe abusers do not really care about it as long as they do not risk anything.
Actually, the old users can use the code if patiently for 14 days. Multiple accounts is very useless if don't have multiple phone numbers to withdraw it. So if the cheater uses the same code number phone with different IPs, of course that makes it suspicious and he will get banned soon.