Controlling Growth of Herbal Lawns

I've read your response to a question about herbal lawns and I'm quite interested [from the Q&A article "Herbal Lawns"]:
"Two herbs that come immediately to mind are wild thyme and yarrow. Both will form a dense turf, and both can tolerate cutting with a lawn mower. Neither is especially strong in shade, however; so you would have plant other herbs in any shaded areas. Thyme and yarrow can tolerate partial shade; you need only consider other herbs where shade persists more half of the day.
"Wild thyme is the only creeping thyme that can be grown from seeds, which is significant if your budget is limited. There are many other low, creeping and spreading thymes, but none propagate by seeds and hence the establishment cost is very dear for large areas. You would need 2-4 plants per square foot to get a decent cover in one year. Seeding wild thyme is by far the smarter way.
"There are many types of yarrows. The one I recommend is the common yarrow, Achillea millefolium. this species is available in seed form. I am especially partial to its lovely show of lacey leaves. When kept low with occasional cutting, it forms a wonderful soft mat that begs of hours of blissful laying about."
My question is: How do you control these lawns so that they don't take over the neighbors lawns as well...and would thyme or yarrow attract any unwanted animals or insects? (We're backing on to a ravine.)

There is always the potential herbs will spread in certain areas where they find the growing conditions to be favourable. Wild thyme does have a history of spreading beyond intended areas where the soil is sandy and dry. For example, in Eastern Ontario, around the town of Perth, wild thyme has become established and is spreading. However, I do not consider wild thyme to be weedy or uncontrollable – at least it is no worse than most turf grasses that also spread. Wild thyme can be controlled easily with occasional cultivation and mowing, where it is spreading to areas it is not wanted.
Yarrow is much the same. Yarrow is not considered invasive but it will establish itself where conditions are suitable for it to do so.
If you are seeding over an area that abuts a neighbour's lawn that is planted with grass, then you need to place a barrier between the thyme (or yarrow) and the neighbour's grass – because there is no way to control either the grass from invading into the herbs, or the herbs encroaching on the grass. Your local nursery establishment can suggest barrier options such as stone, brick, metal or plastic edging materials.
Neither wild thyme nor yarrow are any more likely than turfgrass to attract pests. Pests should not be a worry.