They don't.

Google use spam reports to get an understanding of the current spam techniques to develop the algorithm for automatic detection, ban and reinstatement when clean.

They have stated that they only very rarely intervene manually in incredibly extreme cases.

Don't expect anything to happen from the spam report. I doubt they have the manpower to deal with them all - I guess they gets 1000'3 every day - I also guess a lot are not legitimate (eg SPAM = Sites Positioned Above Mine)

Most do seem to get dealt to eventually as the algorithm is improved .

CBP