Making an ACF field readonly to prevent the users to enter a value.

functions.php

<?php
// Make Custom Field Readonly
function make_price_field_readonly($field) {
	$field['readonly'] = true;
    return $field;
}
// price_field_name is the slug of the Custom Field
add_filter('acf/prepare_field/name=price_field_name', 'make_price_field_readonly');