That's funny...not the fact that the horsetail is acting so aggressive...but your way of describing it.

I had the same thing happen to me years ago. The guy that gave me my first little clump of the stuff warned me that it could spread, so I naively put down one of the flowerbed edger things, thinking that would stop it. And it did for the first year. However, the second year, the stuff decided to take over about a 500 square feet in one giant "gallop". It totally freaked me out, because, with the exception of maybe a really fast-spreading bamboo, I had never seen a plant move that fast. I spent days digging out ever piece of root I could find. Then I went to Home Depot and bought one of those half-barrel pond liners and planted a big clump in that. I used half peat moss and half sand, which it seemed to like just fine. For the next year, Every time I went outside, I'd scout for more Equisetum. Every time it sent up a shoot, I'd place about three to five drops of concentrated RoundUp on it. I think there was still a little that popped up the second year, but not much. It finally gave up and I got my flowerbed back. Now I have two pots of the stuff, one in the ground (the one I planted originally) and one on the back deck. It seems reasonably happy even though it seems to have given up on taking over the state.