Their relentless staring was starting to annoy him.
’OK,’ said Harry quietly, while a moth-eaten troll paused in his relentless clubbing of the would-be ballet teacher to watch them.
But Ron had never endured a relentless campaign of insults, jeers and intimidation.
In fact, Neville barely spoke during the DA meetings any more, but worked relentlessly on every new jinx and counter-curse Harry taught them, his plump face screwed up in concentration, apparently indifferent to injuries or accidents and working harder than anyone else in the room.
The arrival of the Dementors in Little Whinging seemed to have breached the great, invisible wall that divided the relentlessly non-magical world of Privet Drive and the world beyond, Harry’s two lives had somehow become fused and everything had been turned upside-down; the Dursleys were asking for details about the magical world, and Mrs Figg knew Albus Dumbledore; Dementors were soaring around Little Whinging, and he might never return to Hogwarts.
As Harrys eyes became accustomed to the brilliant glare, he saw clocks gleaming from every surface, large and small, grandfather and carriage, hanging in spaces between the bookcases or standing on desks ranging the length of the room, so that a busy, relentless ticking filled the place like thousands of minuscule, marching footsteps.
There are no more uses of "relentless" in the book.