Difficult but not impossible. Here's something i wrote on the subject

http://www.flickr.com/photos/31559373@N00/4757456972/

I found going over the whole of the Horsetail/Marestail with a strimmer then using a weedkiller does work over time. They are a sign of poor wet soil, so improving drainage and adding nutrients would be the thing to do after eradicating the Marestail.

They may generally be a sign of poor wet soil but I look after several gardens which had mares tail (until I eradicated it) and they're all have rich well draining soil

They tend to be more prevalent on acid soils as well, so you could try liming it as well.
You have to be very attentive to digging out as much root as you can get, as soon as you see a new shoot poking through, and before they have a chance to open up, as it's at this stage that they spread their spores, so digging out as soon as you see them is very important, keep doing this and you will eventually eradicate them, although it may take quite a while, Derek.

Good luck! An almost impossible task!