i revoked a write permission form a file.

$ chmod u-w abc

Eventhough i can override the write permission(I can alter and save the file in vi editor using :wq! command). then how linux permission is powerful.

The "!" is to override and ignore complaints. Since the user owns the file he has permissions on it no matter what. Without write permission explicitly given it will complain about attempts to write. So if you try to rm the file it would prompt you but wouldn't prevent you. If you do rm -f (DANGER in this command) on the file you're telling it don't even prompt me - just do it.

If you really want a user NOT to be able to write to a file you should make someone else the owner and then put him in a group that has access to the file and remove write from group and other.