MySQL Mistake with GRANT OPTION
There is no "mass revoke" statement so you best option to change this is to update the users table:

UPDATE mysql.user SET Grant_priv = 'N' WHERE user != 'root';


Not that occasionally, there are users that should have grant privileges (and should have that) which are not named root. On my system, for example, I have debian-sys-maint (which I actually wonder if they should have grant privileges, but that's how it's installed).

Do not forget to reload the privileges after changing the user table:

FLUSH PRIVILEGES;


You can find more information on http://dev.mysql.com/doc/refman/5.5/en/privilege-changes.html.