I checked the below Salesforce Help Link for setup of Extended Mail Merge:- https://releasenotes.docs.salesforce.com/en-us/summer17/release-notes/rn_sales_productivity_ex_mail_merge_setup.htm
But I can't see any user permission which can control the same. I don't want all the users to have the Mail Merge access. Kindly suggest the user permission to restrict the users for the same. Thanks in advance.