Would someone be so kind to help out a beginner?

Lock the gate - they have to ask then

Eventually they will get fed up of having to come and ask, they do learn to be a bit more careful with the ball .... I have found anyway.

Plus if you give it them back after the dog has put holes in it .... they kinda get the idea that their ball needs to stay clear of your garden or else its going to be pretty useless!

Unfortunately your dog then ends up with a bad name, as the ball destroyer .... but at least your not the neighbourhood grump! lol

 

I can't unfortunately. The back gate is shared between me and my neighbour, and he isn't keen on the idea of putting on a lock on it.

I used to have a dog years ago that was the ball destroyer. She was such a lovely, 'helpful' dog. If my mates and I were having a kick about, she would kindly go and retrieve the football if it strayed away from us so we wouldn't have to run after it. She just didn't understand that after she'd retrieved it, it was no good to us anymore and our game would end

 

how about a double barrier? If you have the cash, edible things are usually highly appreciated amongst the midget population. Apple trees might be a bit slow but what about currant bushes? Pretty, stealable fruits, and slightly thorny. The educational value is great, have you ever seen a kid trodding over strawberry plants?

The next layer towards the house could be a fence. Boring but effective.

And for the last defence, Rosa Rugosa is pretty, tolerant, cheap and in the autumn provides berries full of ticling powder for the junior usage.

Or, if you know the local rascals personally, you could always try the "nice cop" option. My fenceside bulbs were saved this year only because I employed the neighbourhood kids in planting the bulbs. Now they keep a keen eye of the danger zone.

Good luck!

 

Gooseberries? Nasty sharp thorns to act as a deterrent and kids being kids, and thus unable to wait for the fruit to ripen, means the double satisfaction that their gurgling stomachs will mean they'll only steal the fruit once.

In our last house the little ***s burnt down 3, 20' long fences until we finally had to resort to the ultimate .... razor wire.

 

As Kristen has already suggested berberis is pretty good, some police force crime prevention officers suggest planting it beneath windows as despite looking quite nice it is just about the nearest natural thing you'll get to razor wire. To put up razor wire or even concealed barbed wire without getting sued by injured burglars you have to errect warning notices just in case a burglar gets scratched.
Pyrocantha has also been suggested it too is pretty tough and thorny but has much nicer flowers and berries. You could try a mixed hedge of both.
Nettles would be a waste of time as they wouldn't be a deterrent during the winter months.