Encroachment is the spread of Japanese Knotweed rhizomes (roots) across property boundaries. If you have exhausted all friendly attempts, then you can follow a course legal action. This should not, however, be the first step.
It is best to try and work with neighbors’ and get a combined effective plan of action to treat the Japanese Knotweed. Where one party has suffered a loss at the expense of the other, mediation between the parties should be given a try before seeking legal redress. Legal costs can be substantial and both parties could end up paying more than they could if they work together in an amicable way.