How to stop weeds from neighbors yard,they are joined

Hello! This is certainly a dilemma, but take consolation in the fact that you are not the first with that particular problem. The best way to control that situation of “mutual weeds” from an adjoining yard is to have not only a fence between properties, but a flagstone or slate barrier on your side of the fence — might have to dig up a few of your plants to install, and also a root barrier. The root barriers are typically made of bamboo and can be purchased online. Good luck.

if you want to really keep their yard separate from yours, you'd have to dig a trench at least a foot deep and 2' wide, fill with concrete and then you'll still have seeds blowing on the wind, carried by birds, etc., sprouting up in your yard .. to stop the runners, the concrete trench would be a permanent solution.

What about some shrubs?

A fence or a line of shrubs can help, some shrubs like Leyland Cyprus grow quite fast. They will give you screening and maybe stop the seeds from spreading as a windbreak.