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.