Sadly there is not real way to figure out what number is behind a "private number". That said there is no way to block a number that you can't find there is the more extreme option of changing your number.
I've seen an application before where you can have the device automatically ignore any anonymous, restricted or blocked calls. I've seen it in the Crackberry.com store. BlackList or WhiteList...something like that. Browse around and maybe you'll find what you're looking for.