I have offered it several times, littleprincesssara.
A ladder would do the trick. One on your side and one on the other side if necessary. Problem solved.
I don't live in a utopia with almost non existent crime. I live in an average American neighbourhood with almost non-existent crime. It seems you resent the life my neighbours and I work to create, Littleprincesssara? I work hard to afford the place where I live. So do my neighbours. We appreciate the nice old trees and the public facilities and the good public transport and we do our utmost to make the community a welcoming and neighbourly place. It is inclusive and very well integrated in every sense. We all appreciate how diverse the place is, and what a nice place it is to bring up children. The community works because people make an effort not to indulge whatever learned and harmful prejudices they may hold and because people who might tend to be marginalised in the UK are very much a valued part of life here. I am speaking about teenagers.
there is no need to be insulting and aggressive towards people who wish to peacefully enjoy their gardens without having balls coming over the fence.
It is unrealistic and actually ridiculous to expect to live your life in an average neighbourhood without having balls coming over the fence.
And just to remind you, the OP wasn't interested in a Quixotic campaign against the balls coming over. She just wondered how she would prevent the knocking on her door by children asking for their ball back. She seems to accept that the ball is going to enter her garden occasionally.
As a 'solution' you offer, 'Tell them to stop knocking and that you'll get the ball if you feel like it'.
It is just plain nasty to punish children essentially for being children by making them wait until you are good and ready to throw their balls back for them, and to refuse to let them climb in to retrieve the balls themselves, all in the spirit of "just because I can", a totally in your face reminder to children that they are powerless, lesser beings and that things that are important to them are way down your list of priorities.
Or in the spirit of "Speak roughly to your little boy, And beat him when he sneezes; He only does it to annoy, Because he knows it teases". Substitute 'kicks the ball over the fence' for sneezing.
If that sort of behaviour gives any of you a sense of satisfaction, I am truly sorry for you.
And incidentally, you can quite easily attach wooden slats or trellises or other weight bearing items to concrete.
Littlecandle:
Do you give your children whatever they want the moment they want it? Or do they sometimes have to wait because you are actually busy?
At an age appropriate level my DCs all learned to wait until I was ready before they could get or do what they wanted to get, or do.
If I said, 'I'll get you your paints and brushes and paper in ten minutes when I have finished cleaning the bathroom,' they would wait and I would do as I promised.
If I said, 'I'll get them whenever I am ready if I feel like it at that point,' they would be rightly upset, resentful, and feeling small and scorned.
If one day I decided to show them how to get their own paints and brushes and how to set up the whole kit and caboodle on the kitchen table with a jam jar of water and a paper towel to wipe the brushes with, and to find and put on their old T-shirts that are kept in the kitchen drawer along with my apron, they could get their own painting stuff for themselves and I could finish up cleaning the bathroom in my own sweet time and everyone would be happy.
See how that works?
There are so many ways to skin the average cat, aren't there? A little thinking outside the box goes a long, long way.