Is there a fence between you & the neighbour? If so, is it low? Does the
fence run alongside a public accessway, footpath etc? How thick are the
saplings?

I am wondering whether in the dead of night you may be able to snap the
*******s off near the base. Do a bit of damage to your garden as well. You
may need to sacrifice a few plants or damage the odd one that was due to
come out or get pruned back (save doing too much damage your side). Next
morning 'discover' the carnage done to your garden and immediately march
round to confront your neighbour. That will put him on the back foot and
reduce his ability to blame you for snapping his trees.

rob