Hi Bruno

I would just recommend to double check that you are certain that your DataMiner Agent won't be affected (i.e. it's truly an old keyspace).

You can verify this by going to the DM agent > System Center > Database, there you'll find the used keyspace.
Alternatively, you can check C:/Skyline DataMiner/Db.xml on the DM Server. The xml will refer to the cassandra DB and the <DB> tag indicates the used keyspace.

Arunkrishna already pointed you towards the CQL version, which is plain and simple:

DROP KEYSPACE <keyspace_name>

But in DevStax, you can also use the GUI:

So bottom-line is really to be certain that the keyspace isn't used anymore.