Are you looking for a method other than drop keyspace?
Okay based on your clarification...
I would say the best way to reset cassandra would be to delete the contents of the <data dir>/data/* <data dir>/commitlog/* <data dir>/saved_caches/*
and then restart the services on the nodes beginning with the seed node and continuing with a 2 minute gap between each node start. That will take the nodes back to a clean state with respect to the data but leave the customizations in cassandra.yaml intact. This would also reset any changes made to the "system" keyspaces at the cqlsh level such as changing a replication factor.
見：http://stackoverflow.com/questions/20784714/drop-all-keyspaces-in-cassandra