Cut off the gall (stem lump) before the wasp emerges. Either prune the entire branch or shaving the exterior of the gall. The galls should be placed into a bag, sealed and put into the rubbish. 

Avoid pruning out the gall wasps in winter

Avoiding high nitrogen fertiliser in spring as this promotes soft sappy growth – just perfect for the egg laying stage.  Feed trees in late autumn and early winter instead.