Fencing type plant

Hi lightyears
How high are you allowed to have a fence?
How about emailing your council executive, or leader - pester pester pester all levels of council - if it is council land they have a duty to protect YOUR rights. maybe try first asking the council official for an address to make an official complaint, may spur someone into action.
you could always find some anti vandal paint (doesn't dry so they may get it on their clothes), but you have to remember yourself of course. maybe paint something and tie string or wire on so you can lift it out of the way and put it back as you leave (not near the path)
leave some (really) bad eggs 'available'
if they take fruit or veg how about 'watering' some specially 
You can buy shed alarms which run off batteries, the one I got for my dads shed made me jump even when I was expecting it. (could you be prosecuted for giving someone a heart attack?)

Mmm, this might be quite a mean suggestion, and i may only be half joking... but poison ivy also grows as a 4 foot tall shrub... causes severe itching.... (mwahahaa) Not sure where you'd get your (gloved) hands on it though.
Or a nice tall row of nettles, with hidden trip wires???

Seriously though, I think something nice and practical like a row of willows, that you can use for training things up-  like tall varieties of peas and beans, or even hops (you could start home brewing) or grapes, or kiwi, or outdoor ridge cucumbers??

a thorny issue indeed...
i would suggest also hawthorn (which supports something like 364 native species) blackthorn (slower growing but you could make your own sloe gin)
 cherry plum is pretty (but more fruit). elder (flowers for cordial or berries for jelly and wine) these are all decidous. holly is evergreen obviously but slow growing. viburnum tinus. roses (rosehip syrup)

i find rosa rugosa makes a really scruffy hedge and spreads out on runners.

 do you have a lockable gate - otherwise, what ever you plant, there still needs to be a gap in it for you (and others) to go through.

failing that what about a bog standard privet hedge. you could 'topiary' some select words of warning to your unwanted visitors

alternatively, you could invite them to your lottie and ask their advice about a few things. they clearly feel threatened by your presence and may be a white flag will help relations. i had the opposite issue... i had an allotment neighbour who came on to my plot, took my rickety fence down and built me a new (and ugly, makeshift looking) one.  clearly i had not been quick enough replacing it (excuse me for prioritising my plants!) he thought he had done me a favour i think.  I was so shocked i couldn't challenge him and just said he shouldn't have (and under my breath -no you REALLY shouldn't have!)

good luck

Roses - get the really thorny type - nothing will get through them, and in the summer you get lovely roses and also the smell - if anyone lives up here in the North, try going to Fryers Nursery in Knutsford in the height of summer, and you can smell the roses virtually straightaway, especially when the wind is blowing in the right direction