Delete unused datafiles from a database

Dear All,

I have a database that is growing so much, we have a lot of applications using that database and most of those applications have been decommissioned (no longer in use). I want to delete the datafiles of those application to reduce the size of the database. The size is huge and the rman backups and the archive logs are generating a lot of files on the server, making the storage management of the server a challenge. We have

Please who can advice on the best way to proceed on this without tampering with the datafiles of the new application. The all the files in the new application is under a schema.