Hi Sandeep,
Unfortunately, that is not how the default value function is set up to work. The default value doesn't lock out other option it just provides a field with an initial value that can then be modified by any users with edit permissions. If you want to prevent users in a specific role from being able to change the default value you would want to change that roles permissions to only be able to View that field and not to Modify it. You can change that within the field settings by checking the box to Restrict access by role and restricting it for a role that should not be able to change from that default value. I hope this suggestion is helpful Sandeep.
