Digital Ocean can't fix this for you. Presumably the other domain belongs to one of their former customers, and they used to have a droplet with that IP addres

Digital Ocean can't fix this for you. Presumably the other domain belongs to one of their former customers, and they used to have a droplet with that IP address. (a) Contact that former customer (hopefully their contact details are in the whois) and ask them to update their dns data. (b) Update your website to return a 400 http error when accessed via that domain name.