Field validation

Hi

I'm trying to do field validation on one of the contact record. Basicly I want give access to modify one field only for 2 users.

Field is check box, and I'm trying to do is bu using field validation function:

(PRE('<field_name>') <> [<filed_name>] AND [<ModifiedbyEmailAddress>] <>"abc@abc.com") OR (PRE('<field_name>') <> [<filed_name>] AND [<ModifiedbyEmailAddress>] <>abc2@abc.com)

but it doesn't seem to work. I start doubt if this can be done for field validation - I know in v20 I could do workflow cancels save, but what can I do for v19? Or maybe is something which can't be done for now?