Hi Srujan,

Its not possible to disable the functionality but we have a workaround for you. 

You can set up a workflow automator that checks if the Source of the ticket is Email and send an email back to the requester asking them to raise a ticket via the portal instead. You can skip the new ticket created email notification to the requester and also spam/delete the ticket raised by email using the same workflow.

We hope this helps.

Regards,

Sanofar

Team Freshservice