Block paving is normally made of concrete. Its rough surfaces make a wonderful home for moss (am assuming the problem is moss rather than mildew) especially in shady areas.

The only foolproof way to prevent re-occurence of moss is to seal the path with either a polyeurethane or acrylic sealer after the blocks have been cleaned and resanded. See my website for details

www.pavingdoctor.com

Jeyes fluid is good in a solution on driveways (brush vigourously before applying and afterwards and then hose off) but I wouldn't recommend it on paths as it should not be applied near bedding plants etc.