welcome to GoY.
oh dear you do have a major problem . they will respond to glyphosate weedkiller but you have to bruise the stems first try walking on them so they get damaged. then spray them. the ones that come up in the grass can be treated the same way but care has to be taken not to damage the grass. use a paint brush/similar to apply the weedkiller.
You will have to keep doing this as horsetails are very resilient.

When they reappear in spring look out for the ones with bare stems and a little "knob" at the top. Remove them as soon as you see them as they contains spores which will make new plants if left (as if you haven't enough problems with the ones you have...)
You could also try digging out every stem you see as soon as you see it - this prevents the plant being able to support its immense deep root system and will gradually weaken it. One of our members is having some success with this method.

Oh dear, I've forgotten the difference between horsetail and mares tail again. Please remind me then I might be able to help with this question.

in the uk it is the same plant. so not to worry Hank.

Not the same plant, Mare's Tail is a water plant seen in ponds and lakes. People just use the name in error, it is confusing! Horsetail is the fern that is a nuisance in the garden.

Was just about to thank you SB when the following reply turned up. I bet Lizzy 71 who asked the question has horsetail ?

When someone asks about it in the garden its pretty well sure to be horsetail unless the garden is a bog...
Just out of interest, these are very primitive plants related to the large ones that fossilised into coal.

I do agree Landgirl but this is the problem with common names. The aquatic mares tail Hippuris vulgaris is a true flowering plant.
Many people call horsetail/mares tail [even the RHS continue to say so too] when they mean Equisetum arvense. Which as you know, is in the same Phylum Tracheophyta as the ferns. They are however in different classes. The Equisetum are a single genus and are considered as living fossils. I was fascinated by them as a young girl 8or 9 yrs old and went to the local museum/natural history site in Sunderland to ask what it was. The gent there was brilliant at explaining it's classification to me and showed me how it was different to ferns and what we would also call the flowering plants. He invited me back several times to look at the fossils they had of the varies plants for each group.
I think Sunderland had such a good collection as it is/was a mining area Stera. I used to have a coal ball but cant for the life of me find it :o(

Right, that clears it up very nicely. So Lizzy 71, if you'd like me to explain how I got rid (almost ) of horsetail, let me know and I'll send you a private reply - as I did to another member a few weeks ago.

Seaburngirl that sounds fascinating. You were very lucky to have a kind curator willing to spend time showing you!
I can remember when I was small and we had coal fires that occasionally you would find a lump of coal that still showed the pattern of the stems on it.

we still have a coal fire and I always look for fossils, no luck. my youngest brother worked down the pit in wearmouth coal fields and would bring 'finds' home for me. that's where I got my coal ball from.