Right. Try:
MERGE DELTA OF TABLE-NAME WITH PARAMETERS ('OPTIMIZE_COMPRESSION' = 'ON')
We discussed this in some detail here: https://scn.sap.com/thread/3637730
You will also probably have to create a savepoint:
ALTER SYSTEM SAVEPOINT
and if you want to clear actual disk space, defragment:
ALTER SYSTEM RECLAIM DATAVOLUME 120 DEFRAGMENT
You could of course just leave HANA to itself, and it will manage everything nicely without you getting involved.