Yes you can put spikes on your fence or wall to stop anything crossing it.
You can even put razor-wire on there providing your fence is more than 2 meters tall and you have put warning notices up.
" manufacturers of these products tell me there is no specific legislation that forbids their use, as long as the wire is installed with reasonable precautions. This means that a material such as razor wire should be clearly visible, and not positioned in such a way that it could act as a man-trap. It should therefore either be up high, on top of a fence, wall, or roof, or else protected by an outer fence.
In other words, it should not be possible for an unsuspecting, innocent party to come into contact with it accidentally. It should also be clearly signed with yellow warning triangles that say "Razor barb - Danger" or "Warning - Barbed wire". Proof of adequate signage should provide a good defence in the event of a civil action from an injured trespasser. "
Quote from this page>
http://www.telegraph.co.uk/property/3351690/On-the-level-what-is-the-law-on-property-security.html
Have a word with your neighbours about the cats...if they are typical cat owners and won't admit their cats are a problem..telll them you're gonna install the spikes first..then razor wire if that don't help.