Cats hate pungent smells so try anything with a strong smell.
Coleus canina is one which you can plant out soon.Put it tubs so you can move it round the garden to foil them .
http://www.hedging.co.uk/acatalog/product_5460 9.html
Those "Get Off my Garden " crystals are good to.Or the aptly named Lion Dung which you can get at garden centres.
I would have a word with the neighbours though ..it's not nice with small children playing in the garden I have stood in it and then tramped it through the house not realising...and it stinks !
If the neighbours won't play ball then get a water pistol and each time you see a cat squirt them with water .It won't hurt them and they may get the message .
Why people can't train them to go on their own patch in a litter tray is beyond me.