ORACLE TABLESPACE USERS GETS 100% USED
I would say that some or all of your application data is likely stored in the USERS tablespace. Don't do anything rash. 

IIRC databases created by the dbca utility have USERS set as the default tablespace. This means that any new users created will store their tables etc. in USERS by default. 

select * from dba_segments where tablespace_name = 'USERS' will give you a starting-point for seeing what is in the tablespace. (It could be a long list.) 

select * from dba_users where default_tablespace = 'USERS' will show you the schema owners which will store their data in USERS by default. 

I suggest that you don't see it as a problem to be solved, but accept that USERS is where the data is stored, and plan for controlled future growth of the tablespace. I have seen a couple of applications which had been installed like this. 

HTH