UNLIMITED TABLESPACE is kind of "wierd" system privilege. It is not granted to any role (and in fact it CAN NOT BE GRANTED to any role!), yet when you grant some powerfull roles (like DBA and I think RESOURCE too - it might even be version dependand AFAIK) to user, Oracle "silently" adds that UNLIMITED TABLESPACE privilege to that user directly. And when you revoke such role from user, Oracle silently revokes UNLIMITED TABLESPACE privilege from user too!

Now, when someone gets UNLIMITED TABLESPACE granted, it doesn't meen taht quotas on all tablespaces are set to unlimited to him. His quotas on tablespaces remain exactly the same - if he had quota 0 on tablespace USERS before, he still has quota 0 on that tablespace now. But when he has UNLIMITED TABLESPACE privilege, this takes priority over any quota settings and therefore he will be able to use space in that tablespace for new extents. However, if you later revoke UNLIMITED TABLESPACE from that user, his quota 0 will be put in effect and he will no longer be able to use any additional available space from tablespace USERS.