Glyphosate has some effect, limited because of the waxy surface of the horsetails and because of some resistance. Try repeated applications a week apart. Or look for Casoron G4 which is supposed to be very effective in one treatment, although I've not tried it. The spores will persist and you will have a problem for a number of years.

i think shrubs n bulbs is right the only thing you can do is keep at it. you can try bruising the plants with a stick before application because the leaf surface is resistant to penitration and try to get really good coverage of the foliage with the spray.
good luck

We had a bed at work infested with marestail last year, the whole area was covered so it looked almost like a lawn!!
Well just about this time last year i carefully walked over it all, gently bruising the stems, its actually quite brittle so you dont want to go too heavy handed and snap the stems. I then sprayed the whole area with glyphosate and sat waited........seemed to get a good "kill" so start of this year as marestail likes wet ground I deeply dug over the bed and weeded out as much of the roots as i came across, thankfully 99% of the roots i found were dead, however we left the bed empty all this year and amazingly only found one shoot coming up in a corner so I'm hoping we have tamed it.

"We had a bed at work infested with marestail last year"

Marestail is a completely different plant (Hippuris vulgaris) - what applies to that won't necessarily apply to horsetails

Resin

Pineresin, while Marestail/Hippuris vulgaris is indeed a completely different plant, the name is very often misapplied to Horsetail. I'm fairly positive that Nbpen's bed of marestail was Equisetum rather than Hippuris as s/he walked over it. Hippuris grows in ponds.

Unless there's something very special about Nbpen...

http://www.gardenadvice.co.uk/howto/gardenplants/weeds/index.html

Equisetum arvense, common names: marestail, horsetail.
Thats why plants get latin names to stop this sort of misunderstanding. I read Equisetum arvense in the original post and knew exactly what the plant was, I was just using "my" name for it: marestail.