Hi , I'm James DVM MRCVS and would like to help you with your problem
Sorry to hear about your inconveniences with the next door cat .
Let me for starter give you some advice as to how to help you own cats feel better
1. Use a microchip recognition cat flap - for ex - http://www.sureflap.co.uk/products/details/1-microchip-cat-flap
This will make sure that only your two cats can enter the how and no other
2. use a feliway diffuser - its an amazing product which spreads calming pheromones around the house leaving your cats calm and happy (would be most beneficial to your little one ) - http://www.feliway.com/uk
3. sadly there is little that you can do in order to make sure you next door cat stops visiting . you might want to work it from a different approach - perhaps place some food done for this poor neglected cat as well , location that he would be sure to reach on the one hand and would be out of site from your two cats. not other human measure would stand between a hungry cat and food so all the means you have sued so far will not work .
Toms are getting into fights on daily basis and the there is little one can do against that once two males (even when castrated) mean to keep their territory .
I hope this helps .
If you have any more question , please ask .
Please remember to rate our conversation.
Hello again, Just wanted to know how things are and if there is any improvement?