Ash trees seeding and taking over our local gardens

A neighbour has a large back garden. Many years ago the previous resident planted 4 sapling trees. The only surviving tree is an Ash which is now 20 feet plus in height. It's very beautiful but it has seeded in almost every back garden including my own. One seeded and grew very rapidly through some very old cracked paving slabs at the side of my greenhouse. I cut the Ash tree growth back for 3 years right down to ground level. The tree continued to flourish and grow back with each year. Eventually, when about to redesign the garden and have the greenhouse (a 40 year old building inherited when I bought the property) pulled down and the Ash tree had to go. Took up the old cracked pavers, attempted to dig out the Ash tree roots, impossible. Did some research, ended up purchasing some poison and drilled a hole deeply down in the centre part of the trunk, with care put the poison down the drill hole aperture. It must have worked because the tree never grew again. Had to leave the roots in Wasn't going to be planting in that area as new paving was to be laid over that area. It worked.
In the following years several new growth Ash trees began to appear in surrounding neighbours gardens.
To date we have the original central "Mother" Ash and her related surrounding ash trees now number at 7 (known at present). 2 in my immediate next door 84 year old disabled neighbours garden. One of which has killed off his hydrangeas. Another which is right up against a tall brick built garden outer wall and damaging the base of the wall. He is going to have the Ash trees removed as soon as his son gets round to doing it.
Late this Springtime I discovered an Ash had seeded in a huge planter tub at the rear of my garden among some Salvias I'd had for about 4 years and killed them off and that Ash grew 2 to 3 feet tall within 3 months. It's still there, but sadly not for much longer because as much as I am passionate and love nature with any more Ash trees seeding successfully looks as if we could end up with a forest of them (not too adviseable on a housing estate).
Yesterday, while dead heading some roses that grow approx 1 foot away from my fence bordering onto my other next door neighbour's garden I discovered yet another Ash beginning to grow, pushing it's way through a 1cm gap in the vertical wood upright's of the fence. On inspection I immediately appreciated that the roots were situated on my neighbour's side of the fence, in her garden, and not mine. It was just a young leafy branch about one foot above ground level that had pushed it's way through to my garden from hers.
She is not a gardener and her garden is a corner triangular plot and very small. She has nothing living (no plants, bushes, plant pots, nothing at all) in her garden other than decking, some paving with gravel infill around the perimeter of it and that's where the Ash is growing up through the uncovered shallow gravel from the soil.
Furthermore it was only 2 month ago that she had the garden redesigned and the decking, paving and gravel laid. The Ash tree is growing approx 10 feet away from her kitchen window. I also happen to know there's drain works near to where the ash has rooted which inter-connectedly serve her property and mine.
I have a very well stocked and well maintained garden and without me taking down the fence and interfering with her newly laid gravel and paving I can't dig out the roots from my side. As it's a young-new growing Ash what is my best plan to get rid of it before it rapidly continues to grow and cause damage to my fence, my roses and other plants and bushes - and possibly our drains, etc. Have pointed out the Ash growth to my neighbour who has no knowledge of gardening, no interest in nature (believe me).
Have told about Ash trees and their growth rate and the damage they may cause so close to the houses (especially hers)... Should I attempt to poison this young Ash or should I just keep cutting it back as it grows through my fence - but doing that I do appreciate that the roots will be continuing to spread into my garden and will kill off my own well tended plants? All constructive advice will be gratefully received. Thank you and Happy Gardening!

 

This is what ash trees do. I live next to an ash wood. Just keep pulling them up if you don't want them, not much else you can do apart from cutting them down.

 

I pulled out 16 baby Ash trees from a tiny part of our garden yesterday. My wife went over the same patch 2 hours later and pulled out a further 10. The Ash tree in our hedgerow is probably 60 feet tall and produces a humongous number of seedlings. All part of the gardening experience.
Tiny ones pull out easy, slight bigger ones are impossible, 2 feet tall ones are easy again.

 

SBK weedkiller does the trick!

 

Given the tricky situation, I'd paint it with SBK as suggested above.