As far as Dataiku DSS is concerned, simply execute
make a backup of the DATA_DIR folder, then delete it. Finally, delete the INSTALL_DIR, i.e. the folder created when you uncompressed dataiku-dss-VERSION.tar.gz
You might want to check if datasets have been stored outside of managed datasets (for instance on a SQL DB, on FTP, on Hadoop, etc.) and see if you still need them.