Available in RC build.
Currently supporting the exclusion operator (-) on only string facets. Usage:
filter_by=author:- JK Rowling (excludes a single value)
filter_by=author:- [JK Rowling, Enid Blyton]
(excludes both authors)
If you want to include some and exclude others you can have two filter_by clauses separated by && e.g.
filter_by=author:- [JK Rowling, Enid Blyton] && author: [JRR Tolkien, John Grisham, George Martin]