Posted: 21/02/2014 at 18:26
I think you should tell them very politely that anything thrown into your garden will be handed back to them at a time that is convenient, in other words at your convenience, not theirs. So, for instance, if the ball comes over six times in half an hour you are not responsible for giving it back straight away or at any particular time. But you should not play into their hands by taking the ball hostage.
Make it clear that they are in no circumstances permitted to come into your garden and help themselves, and that if they do you will seek legal advice on trespass.
You should also keep a written record of any incident that causes you inconvenience (digital photos have a date on them) so that should things get worse you will have evidence if you choose to go to the local PCSO for advice.
Does their neighbour on the other side have similar problems?