If they are encroaching onto your property, and if he will not cut them back or come onto your property at your invitation to do so, then you are legally entitled to cut them back yourself - but you must give back the cuttings.
As for spraying the roots, well, what you do on your side of the property line is your business, as long as you're not doing anything that's illegal or will endanger animals.
Of course, you could go into the roof space and spray the offending growth with a systemic weedkiller. Not your problem if it works its way through the system.