It is impossible, using the roles system, to prevent a user from updating (or editing, or even viewing) his own profile. For example, see discussion http://moodle.org/mod/forum/discuss.php?d=77540 where users describe various combinations of capabilities and contexts in which they have been applied.
A possibly-related issue,
MDL-8820 "moodle/user:editprofile not working in user contexts," was closed. It seems to address only the ability of users to update/edit/view OTHER users profiles, not their own.