Thanks Klas!
I need to unset a field in content view for some groups and on mobile phones, I've been trying to play with BeforeRender but cannot get it work, I'm still not clear how to control the restrictions, you gave this example $fields['yourf_field_name']->restricted = 1;
What does mean 1 in your example?
For my purpose in admin panel I have set one restriction - "groups permissions", now how to set the second restriction for mobiles devices?
Thanks for any help!