Sometimes it's useful that a field cannot be modified/deleted in the class editor. Especially if a class is created by a plugin.
pimcore offers the possibility to lock a field programmatically, you can call the method setLocked() on every Object_Class_Data object.
Example
The following example will lock every field inside the class with the ID 7.
$class = Object_Class::getById(7);
$fields = $class->getFielddefinitions();
foreach ($fields as $field) {
$field->setLocked(true);
}
$class->save();