customize permission in Cassandra
I don't think this is possible natively. The Cassandra role-based permissions as shown here show that granting a user MODIFY includes both UPDATE (which is effectively INSERT) and DELETE. You can't select which one you want.

Check out this blog entry I found, though and see if there's anything useful there. It looks like you could do read or write only permissions. Maybe if you can do write-only permissions you can limit the user from being able to delete a table or keyspace.

Hope this helps point you in the right direction.

-em