You can login to Facebook via and see if there is a setting to disable request. I'm not sure that you can since Facebook is all about making connections. Still doesn't hurt to check and try.
By default, anyone on Facebook can add you as a friend. If you’d like to change who can send you friend requests:
Click in the top right of any Facebook page and select Settings
Click Privacy in the left column
Click Edit next to Who can send you friend requests?
Select an audience from the dropdown menu