...three fully grown teenagers start football and within minutes there are four balls in my garden.
I don't think people are quite getting this.
It's not the 'odd football' that lands in your garden 2 or 3 times a week which few people would mind.
Teenage boys' brains are not wired to self-regulate. Their bodies are almost mature but their brains are not. They probably have no care or concept that their kickabout is causing problems for others. I doubt they are even playing football, they're just kicking it as hard as they can for the hell of it.
It's up to the adults to moderate their behaviour - but as the father joins in, I doubt there's much 'maturity' there either.
People are allowed to enjoy their gardens, we know that, but there's a point at which one person's enjoyment becomes a bloody nuisance. Kicking balls with such force that they continually land in someone else's garden, playing very loud music with windows wide open, individuals 'partying' in their gardens, shouting and screaming until the small hours is not people enjoying themselves, it's plain anti-social behaviour. The family sound like they are self indulgent, ignorant and obnoxious neighbours... they probably think it's a good laugh.
The OP has posted before about this - her neighbours appear indifferent. They simply don't give a damn.