bumph is an alternate spelling of the word bumf, which means "useless or tedious printed material" (Oxford English Dictionaries). People often feel that the bumph they receive in the post, with magazines or inside newspapers is an annoying distraction - a bit like hitting a Bludger into the crowd to stop the match.
From the Web
WizardingWorld.com (Pottermore) feature: Quidditch Through the Ages
Writing by J K Rowling on Imgur (Daily Prophet coverage of the 2014 Quidditch World Cup, originally from Pottermore): History of the Quidditch World Cup
Harry Potter Wiki: Bumphing