I've heard that if you have it on your bit of land it's all part of one big plant, so if that's the case there doesn't seem to be much choice but to live with it. On the bright side, when I took over my plot it was overrun with the stuff. Eight years on, with no more than average care, it's hardly a problem, giving credence to the claim that it prefers uncultivated conditions. I haven't killed it, it's just gradually relocating to find a more suitable place to reproduce, i.e. the next plot. ;D Btw there seems to be two distinct breeds - horsetail is not the same as marestail, or at least their Latin names are quite different, if anybody can be bothered to look it up (I think mine's the one that ends in 'arvensis'). Whatever it's called, it hasn't gone away by reading about it.
Geoff.