It appears that Astyanax does not currently support the slice delete functionality that is a fairly recent addition to both the storage engine and the Thrift API. If you look at the thrift API reference: http://wiki.apache.org/cassandra/API10 You see that the delete operation takes a SlicePredicate, which can take either a list of columns or a SliceRange. A SliceRange, could specify all columns greater or less than the column you wanted to keep, so that would allow you to do two slice delete operations to delete all but one of the columns in the row.
Unfortunately, Astyanax only has the ability to delete an entire row, or a defined list of columns and doesn't wrap the full SlicePredicate functionality. So it looks like you have two options: 1) See about sending a raw thrift slice delete, bypassing Astyanax wrapper, or 2) Do a column read, followed by a row delete, followed by a column write. This is not ideally efficient, but if it isn't done too frequently shouldn't be prohibitive. or 3) Read the entire row and explicitly delete all of the columns other than the one you want to preserve.
I should note that while the storage engine and thrift API both support slice deletes, this is also not yet explicitly supported by CQL.
I filed this ticket to address that last limitation: https://issues.apache.org/jira/browse/CASSANDRA-6292