Here’s what I did, since most of the time there are multiple admins on the site but I don’t want any besides myself editing the Custom Fields:
add_filter('acf/settings/show_admin', 'my_acf_show_admin');
function my_acf_show_admin($show) {
// provide a list of usernames who can edit custom field definitions here
$admins = array(
'jason'
);
// get the current user
$current_user = wp_get_current_user();
return (in_array($current_user->user_login, $admins));
}