Please see https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/only-admin/
Go to Front End PM PRO > Settings > Recipient and set following
1. Max recipients – 0 (Zero)
2. Can send to admin – checked
3. Admins – set your admin name and username (only one)
4. Can send to group – unchecked
Then save changes.
Let me know.