This can be done using Helpdesk Restriction (available from the Estate plan). You can enter all the domains that are allowed to create tickets and sign up/log in. You can whitelist a maximum of 40 domains; however, tickets created by an agent on behalf of a customer whose domain is not whitelisted (or tickets created through the chat or phone channels) will not be dropped and the contact of the ticket requester will be whitelisted. This feature is available at the Estate and Forest plans in Freshdesk.
Another workaround would be to create an automation rule under Admin > Automations > New Rule under the Ticket creation tab to delete the tickets based on the email domain.