You can create a custom role and manage the Ticket access for the agents assigned to the role under Permissions.
To disable the option for agents to link tickets,
- Go to Admin > Teams > Roles
- Create a New Role or click Edit next to an existing custom role.
- Scroll down to Permissions.
- Under the Tickets tab, uncheck the box next to Create a linked ticket.
You can now, assign this role to all the agents who should not have access to create linked tickets.