PP isn't about rewarding behaviour. It's about trying to improve the life chances of some of the most disadvantaged students - in most schools the most underperforming group are boys, specifically white working class boys.
PP is an attempt to make up for lack of opportunity and to put them on an equal footing. Sadly, the PP money isn't enough to make the difference and schools have to try and show impact whilst spending little money.
You say that these boys weren't interested in learning but you really need to look at what has happened to bring them to that point and look at what can be done to compensate for it.
It brought it home to me when I was a governor with responsibility for PP while at the same time seeing my students through 6th form and uni applications. I would see students at school whose parents showed little interest in them, either because they didn't care or weren't able to, whose parents didn't know where they were, who they were with, had no idea of their performance at school, didn't take any interest in their education or their prospects. I looked at my own children and how much time it was taking to visit unis, to chat about what they wanted to do, how they could achieve it, what support they needed, going to parents evenings, supporting after school activities, providing resources and opportunities for them. That's what PP is meant to do.
The education system is failing massively. Head teachers are being forced into defensive mode - their jobs are under threat from factors largely outside of their control. OFSTED can act on a whim and plunge a school into special measures by ignoring any positive aspects and concentrating on a tiny number of negatives and so of course HT are going to concentrate on OFSTEDs pet issues. The problem is these change like the weather. We have to have some stability so that schools get the chance to build on good practice. At the moment nothing gets the chance to embed because what was important yesterday has to be ignored tomorrow.