They could run a length of ethernet cable down the garden and use a PoE live streaming camera infrared camera with software recording just the motion activated bits. Or a battery powered infrared motion activated camera recording to Sd-card?
Assuming it is a publicly owned path then you could petition the council for a gating order to tackle the antisocial behaviour. Council installs a metal gate at either end of the path and then provides keys to each of the properties accessed by the path.
Or if it is owned by the freeholders and not a public right of way then you could do the same by consensus of the freeholders, or just pretend it is a council matter and dupe them into putting in the gate anyway as it is not uncommon for them to do this despite the path being private land.
or if it is owned by freeholders and not a public right of way then you could by consensus extend your gardens across the access.
or if it is owned by freeholders and is a public right of way then you could still do either of the above if you can successfully process a stopping up order.