Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
Currently that was no option to disabled that add friend button from backend, but you can apply this following css into this template file location, so it will hide this add friend button on the page.
JoomlaFolder/templates/YourCurrentTemplate/html/com_easysocial/css/custom.css (you have to manually create these new folder and file from your template folder)
#es .btn-group.btn-group--es-friends a[data-es-friends-button] {
display: none !important;
}