Actually if he keeps on encroaching I'd speak to him person and advise him firmly that he wouldn't like it if you started taking his plot would he?
I most certainly wouldn't put salt etc down- thats just wrong and when hes plants etc start dying it'll escalate matters meaning you could be ejected.
If he still moves his borders after talking to – move his 2ft into his patch .
If hes older/elderly there isn't much more you can do. I was happily swimming at my local pool when a lad dived in and phyiscally shoved me out of the way. I spoke to the pool attendant who said 'yep we have lots of complaints about him, he sees that part of the pool as his and can get very upset' (he was circa 65).
What could I do? Nowt. It'd be a petty and pointless battle that he'd win.