[dotBen]

Simple - you give candidates assignments you've designed solely for interviewing. Ones that you clearly state up front are not real, not based around current project deliverables and commit that the code will ever be used in production.

Then it's no different to the rest of the candidate's time spent interviewing - it's cost of doing business that no one expects to be remunerated for.

The other benefit is that you then keep a benchmark of deliverables by having each candidate complete the same task (you have to change tasks over time as the details of the project get leaked/discussed).