How can I keep my neighbors weeds from invading my yard?

Get a wall put up and weed killer

Vinegar the creeping charlie.

There is nothing you can do but catch it and remove it as it comes into your yard unless the neighbor is willing to treat for it or let you do it. You can't stop it at the line as their are still plenty of mother plants on the other side of the line to throw off roots that will invade your space. Beware of using anything that will possible kill your grass and prevent you from regrowing anything in that space. The only way to at least slow it down would be to use a grass friendly weed killer and spray it a few feet over onto their property. That would at least give you a break for a while.

We made a small trench so his wire grass would not invade our lawn. When it creeps in the trench we use roundup

Try using vinegar!