This is one of those simple questions that probably doesn't have a simple answer. You could get an opinion from a solicitor & - if that was to the effect that you could stop it - a solicitor's letter to the neighbour might have the desired effect. If it didn't, you would have to go to Court for an injunction. (All this is on the assumption that you would be within your rights to stop it.)
A lot of hassle & cost - is it really worth it? If you succeeded they could put up a fence or other support a few inches from your wall, grow things on it & there would be nothing you could do. It might be better to come to an agreement with them about what sort of thing they can grow, pruning etc., & about removing them if you needed access to the wall for repairs etc.