Got too many tombstones? This blog post will talk about how to deal with tombstones once you already have them. For more information about tombstones, check out this post: Examining the Lifecycle of Tombstones in Apache Cassandra.
Click through for several techniques for handling tombstoned records in Cassandra. In SQL Server, with columnstore indexes, the prevention advice is similar (avoid deletion or updating of data) but the treatment options are quite different.