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.
Lexicon list of Quidditch fouls
From the Web
Writing by J.K. Rowling on Pottermore: History of the Quidditch World Cup
Pottermore feature: Quidditch collection
Harry Potter Wiki: Bumphing